photoionization of ch3i mediated by the c state in the visible and ultraviolet regions

RAPID COMMUNICATIONS IN MASS SPECTROMETRY

Rapid Commun. Mass Spectrom. 2005; 19: 1522–1528

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/rcm.1951

Photoionization of CH3I mediated by the C state

in the visible and ultraviolet regions

P. Sharma1, R. K. Vatsa1, B. N. Rajasekhar2, N. C. Das2, T. K. Ghanty3

and S. K. Kulshreshtha1*1Novel Materials and Structural Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India2Spectroscopy Division, Bhabha Atomic Research Centre, Mumbai 400 085, India3Radiation Chemistry and Chemical Dynamics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

Received 1 March 2005; Revised 31 March 2005; Accepted 31 March 2005

Three/two-photon resonant multiphoton ionization (MPI) of the CH3I monomer has been studied

in the gas phase at 532 and 355nm using time-of-flight mass spectrometry. Under low laser intensity

(�109W/cm2) the mass spectra showed peaks at m/z 15, 127 and 142, corresponding to [CH3]þ, [I]þ

and [CH3I]þ species, at both these wavelengths. The laser power dependence for [CH3I]

þ, [I]þ and

[CH3]þ ions showed a three-photon dependence at 532nm. For the same three ions, photoionization

studies at 355nm gave a power dependence of 2. Both these results suggest that a vibronic energy

level at �7 eV, lying in the Rydberg C state, acts as a resonant intermediate level in ionization of

CH3I. In the case of 355nm, with increasing intensity additional peaks at m/z 139 and 141 were

observed which could be assigned to [CI]þ and [CH2I]þ fragments. In contrast, for high intensity

radiation at 532nm (�2� 1010W/cm2), only the [CI]þ fragment was observed. At these wavelengths,

fragment ions observed in mass spectra mainly arise from photodissociation of the parent ion.

Experiments at another wavelength in the visible region (564.2 nm) confirmed the results obtained

at 532nm. In order to assess the role of the A state in these MPI experiments, additional experi-

ments were performed at 266 and 282.1 nm, which access the A state directly via a one-photon tran-

sition, and showed absence of a surviving precursor ion. Reaction energies for various possible

dissociation channels of CH3I/[CH3I]þ/[CH2I]

þ were calculated theoretically at the MP2 level using

the GAMESS electronic structure program. Copyright # 2005 John Wiley & Sons, Ltd.

Photochemistry and spectroscopy of methyl iodide (CH3I) in

the ultraviolet and vacuum ultraviolet (UV-VUV) spectral

regions have been extensively investigated,1–7 making it the

most studied molecule of its class. CH3I has also been used as

a model system for theoretical understanding of alkyl halide

photodissociation, as its dissociation dynamics vary with the

excited electronic states, i.e., the valence A state and the

higher energy Rydberg B, C and D states. The well-known

A state (comprising three components 1Q(E), 3Q0 (A1) and3Q1 (E)) of CH3I lies in the range 330–210 nm with a maxi-

mum at 260 nm corresponding to the excitation of a non-

bonding 5pp iodine electron to the s* antibonding C-I orbi-

tal.3 Below 210 nm, the Rydberg B and C states, which corre-

spond to excitation of an iodine 5pp electron to the 6s

Rydberg orbital, are observed.8 The B state (6s 2E3/2) lies in

the range 201–190 nm, whereas the C state (6s 2E1/2) lies in

the range 184–172 nm. Excitation to the A state results in

rapid direct dissociation producing CH3 and I (2P3/2 and2P1/2) on a time scale of �100 fs.9 CH3I molecules excited to

Rydberg B and C states have been shown to undergo fast

predissociation via the A state.

Although dissociation in the A, B and C states is very fast it

has however been observed that, under high laser intensity,

excitation from these dissociative states can take place

leading to ionization. This competition between photodisso-

ciation and photoionization is inherently interesting, as it is a

multicontinuum and multielectron process. On irradiating

CH3I molecules with intense coherent radiation, multiphoton

absorption opens the possibility of ladder-switching and

ladder-climbing mechanisms. Even though absorption of a

single photon may be sufficient to dissociate the molecule,

above-threshold absorption of additional photons can lead to

ionization and dissociation on a higher potential energy

surface.

The existence of several different spectroscopically acces-

sible excited electronic states of CH3I, and their different

dissociation/ionization dynamics, makes CH3I an interest-

ing molecule for photochemical studies. Accordingly, several

photoionization studies have been conducted for this

molecule using laser pulses of varying pulse duration

(femtoseconds to nanoseconds) at different wavelengths. In

the dissociative ionization using 50 fs pulses at 790 nm,10

Coulomb explosion of CH3I was observed at intensities of

Copyright # 2005 John Wiley & Sons, Ltd.

*Correspondence to: S. K. Kulshreshtha, Novel Materials andStructural Chemistry Division, Bhabha Atomic ResearchCentre, Mumbai 400 085, India.E-mail: [email protected]

1016 W/cm2 and, in addition to [CH3I]þ, [I]þ and [CH3]þ,

multiply charged ions such as [C]4þ and [I]7þ, corresponding

to removal of all the valence electrons, were also observed.

Using 200 fs pulses in the UV region, Lehr et al.11 observed

mainly singly charged ions such as [CH3I]þ, [CH2I]þ, [CHI]þ,

[CI]þ, [HI]þ, [I]þ, [CH3]þ, [CH2]þ and [CH]þ. Picosecond

ionization studies at 532 and 1064 nm produced similar

fragment ions as in the fs studies but with lower kinetic

energy.12 On the nanosecond time scale, depending on the

wavelength, multiphoton ionization (MPI) and fragmenta-

tion processes in CH3I resulted in nearly complete loss of the

parent ion.13,14 As a result, peaks due to [I]þ/[CH3]þ and a

negligibly small peak due to [CH3I]þ were observed.

To the best of our knowledge there are no reports on

photoionization studies of CH3I using nanosecond pulses at

532 nm. In this work, we report the C state mediated

photoionization of CH3I at 532 and 355 nm. The main

objective of this study was to investigate the photochemical

behavior of the excited molecule when a given energy level is

accessed using different numbers of photons. To understand

the role of the A state in these MPI experiments, additional

experiments at 266, 282.1 and 564.2 nm were performed.

EXPERIMENTAL

Photoionization experimentsDetails of the experimental setup used for this study have

been described in earlier publications.15,16 The frequency

doubled (532 nm), tripled (355 nm) and quadrupled

(266 nm) output of a Nd:YAG laser (6–8 ns pulse width)

was used for excitation and ionization. Experiments at 532

and 355 nm were performed in the intensity range

�1� 109 W/cm2 (low intensity) to �2� 1010 W/cm2 (high

intensity). To generate wavelengths in the 564 nm region, a

dye laser (Quantel TDL 70) with rhodamine 6G dye was

pumped by the second harmonic of the Nd:YAG laser. The

output of the dye laser was frequency doubled in a KDP crys-

tal to obtain the wavelength in the 281–284 nm region.

A pulsed valve (0.6 mm nozzle diameter and 300ms pulse

duration) was used to introduce the CH3I sample into the

expansion chamber. The beam of CH3I was skimmed at a

distance of 3 cm from the pulsed nozzle, and entered the

ionization chamber where CH3I molecules were multi-

photon-excited and ionized. The ions were accelerated and

guided into a 100 cm field-free region using a double-

focusing Wiley-Mclaren assembly, and detected using a

channel electron multiplier (CEM) detector. The ion signal

from the CEM was amplified using a fast-preamplifier and

transferred to a digital oscilloscope for averaging, and further

processed on a computer. The mass resolution of the

instrument is �300.

Care was taken to prevent formation of clusters by varying

the duration of pulsed valve opening and the back-up pres-

sure. Initially, strong signals due to [(CH3)2I]þ and [I2]þwere

present in the mass spectra, but, by increasing the duration of

pulse valve opening and by ionizing at the leading edge of the

molecular beam expansion, which has low sample density,

formation of clusters was avoided. The present studies were

conducted under experimental conditions where no evi-

dence for cluster formation could be observed.

UV-VUV photoabsorption experimentsThe gas-phase absorption spectrum of CH3I was recorded

over the range 110–370 nm using the photophysics beam-

line at the Indus-1 synchrotron facility (450 Mev storage

ring, 32 MHz, 512 ps pulses) at the Centre for Advanced Tech-

nology (CAT), Indore, India. The beamline is based on a 1 m

Seya-Namioka monochromator operating in the UV-VUV

region with a resolution of �0.1 nm.17 In order to record the

UV and VUV spectra, CH3I was made to flow at low pressure

through the cell. The pressure of CH3I in the cell was con-

trolled by cooling the sample and introducing the vapor

into the cell via a leak valve. Absorption in the UV-VUV

region was detected with the help of a quartz window coated

with sodium salicylate and placed in front of a photomulti-

plier tube. Since the absorption cross-sections in the UV

and VUV regions differ considerably, different pressures

were used to record the spectra in these regions.

Theoretical calculationsTo substantiate the experimental results, theoretical investi-

gations were performed to study the energetics of various

possible channels for the dissociation of the [CH3I]þ ion.

For this purpose, ground-state geometries of all the possible

ionic and neutral fragments, i.e. CH3I, CH2I, CHI, CI, CH3,

CH2, CH, and H2, were optimized without any symmetry

constraint using Moller-Plesset second-order (MP2) pertur-

bation theory. At this level of theory energies were also calcu-

lated for hydrogen and iodine atoms and iodine atomic ion

(Table 1). The augmented correlation-consistent polarized

valence triple-zeta basis sets (aug-cc-pVTZ) of Dunning

were employed for carbon and hydrogen atoms.18 For the

iodine atom, the SDB-aug-cc-pVTZ basis set of Martin and

Sundermann,19 along with the relativistic effective core

potential of Bergner et al.,20 were used. For all the optimized

structures vibrational frequencies were calculated, and it was

found that all the optimized structures are true minima in the

Table 1. Total energy of fragments and ions calculated at

the MP2 level using augmented correlation-consistent

polarized valence triple-zeta basis sets (aug-cc-pVTZ) of

Dunning

Sr. No. Fragment/ion Total energy (a.u)

1 CH3I �51.1564712 CH3Iþ �50.7990123 CH2I �50.4836144 CH2Iþ �50.1821305 CHI �49.8092036 CHIþ �49.4862817 CI �49.1717308 CIþ �48.8713879 H2 �1.165023

10 H2þ �0.602302

11 H �0.49982112 2I �11.31008113 3Iþ �10.93119914 1CH2 �39.03441215 3CH2 �39.05758216 2CH2

þ �38.67796317 CH3 �39.73831818 CH3

þ �39.380956

Copyright # 2005 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2005; 19: 1522–1528

Photoionization of CH3I in the visible and UV regions 1523

respective potential energy surfaces. For a particular reaction

channel, the energetics of the reaction (DE) was calculated as

the difference in the total energy of the products and the reac-

tants. All the calculations in this work were done using the

GAMESS electronic structure program, which did not

involve spin-orbit coupling explicitly.21

RESULTS AND DISCUSSION

UV and VUV absorption spectraA typical absorption spectrum of CH3I is shown in Fig. 1. The

inset in Fig. 1 shows the A state with a maximum near 260 nm.

No detectable absorption could be observed at 355 nm even

for pressures of 10 Torr. The spectrum shows continuous

absorption from 122 nm and below; this is due to the ioniza-

tion of neutral CH3I to the [CH3I]þ (2E1/2) state corresponding

to 10.162 eV, in good agreement with the literature value

of 10.164 eV. In the region of 201–170 nm there are pure

Rydberg states, as can be inferred from the sharp nature of

the absorptions and nearly zero background absorption.

However, below 170 nm, sharp states riding on the continu-

ous background absorption can be seen in Fig. 1. These are

mixed states that have valence as well as Rydberg character.

Mass spectra and power dependence of fragmentions at 532nmCH3I is transparent to low-intensity 532 nm radiation. How-

ever, at this wavelength, the molecule can be excited to the

dissociative A state or Rydberg C state by absorption of two

or three photons, respectively. Figure 2(a) shows the mass

spectrum obtained on photoionization of methyl iodide at

532 nm; the m/z values correspond to the presence of

[CH3]þ, [I]þ and [CH3I]þ ions. The nature of the spectra and

fragmentation pattern did not change much with increasing

intensity, except that, under high intensity conditions

(�2� 1010 W/cm2), a small signal at m/z 139 due to [CI]þ

was also detected.

In order to understand the mechanism of the formation of

[CH3]þ, [I]þ and [CH3I]þ ions, the laser power dependence of

these ions was measured at 532 nm. The ln-ln plots of laser

energy vs. ion signal for 532 nm photoionization are shown in

Fig. 3(a). A laser power dependence of�3 was obtained for all

the ions ([CH3]þ¼ 2.3, [I]þ¼ 2.7 and [CH3I]þ¼ 2.3). The

ionization potential (IP) of CH3I is 9.54 eV. In the absence of a

resonant intermediate state, a five-photon dependence is

expected for the parent ion, whereas, for appearance of

fragment ions ([CH3]þ or [I]þ), at least six to seven photons of

532 nm have to be absorbed. However, the experimentally

measured value is nearly 3, showing that a resonant state lies

at the three-photon level and subsequent steps leading to

ionization are partially or fully saturated. Our results suggest

that neutral CH3I is excited to a resonant vibronic level

(6s (2E1/2, 2v2, v2þ v3)) (S. Eden, P. Limao-Vieira, S. V.

Hoffmann, N. J. Mason, personal communication), in the

Rydberg C state, which lies at �7 eV (Fig. 4), by a three-

photon process, and subsequently is photoionized by two

additional photons, either stepwise one-photon via a 9.32 eV

state or a direct two-photon process. The former process will

dominate since, in the region of 9.32 eV, CH3I has continuous

broad absorption which is characterized by mixed valence

and 8s Rydberg state character (see Fig. 1), allowing single-

photon processes to override the two-photon absorption.

Prior to ionization, CH3I has a total energy of 11.65 eV (�5 hn),which is sufficient to produce [CH3I]þ(X) ions in the 2E3/2

state (IP¼ 9.538 eV) as well as the higher lying 2E1/2 state

(IP¼ 10.164 eV). MPI and photoelectron spectroscopic

studies have shown that there is no preference for either

state, and these two states are produced in nearly equal

abundance.11,22,23

210 200 190 180 170 160 150 140 130 120 110

220 240 260 280 300 320 340 360

A state

Wavelength (nm)

Ab

sorp

tion

8s Rydberg/valence state(9.32 eV)

CH3I+

(2E3/2

)

CH3I+

(2E1/2

)

6s(2E1/2

)

6s(2E3/2

)

CH3I

Wavelength (nm)

Abs

orpt

ion

Figure 1. Absorption spectrum of CH3I in the UV-VUV

region (20mTorr).The origin of B and C states, 9.32 eV

valence/Rydberg state, and the ionization threshold for the

two ionic states, are shown. The inset shows the absorption

in the A state which was recorded at a pressure of 140mTorr. 20 40 60 80 100 120 140 160 180 200-0.00020.00000.00020.00040.00060.00080.00100.00120.0014

20 40 60 80 100 120 140 160 180 200

0.0000

0.0002

0.0004

0.0006

0.0008

20 40 60 80 100 120 140 160 180 200

0.00

0.01

0.02

0.03

0.04

(c)CH3I at 355 nm,2 mJ

Inte

nsity

(arb

.uni

ts)

m/zIn

tens

ity(a

rb.u

nits

)

(a)CH3I at 532 nm,8 mJ

CH3I at 266 nm, 10 mJ

inte

nsity

(arb

.uni

ts)

(b)

Figure 2. MPI mass spectra (a) at 532 nm at an energy of

8mJ/pulse, (b) at 266 nm at an energy of 10mJ/pulse, and (c)

at 355 nm at an energy of 2mJ/pulse.

1524 P. Sharma et al.


An energy level diagram showing the A, B and C states of

neutral CH3I and X, and the A, and B states of the [CH3I]þ ion,

all relevant to the present work, is given in Fig. 4. For [CH3I]þ,

the origin of the A X transition lies at 19409 cm�1 for 2E3/2

and at 14359 cm�1 for (2E1/2).24 Thus, single-photon absorp-

tion at 532 nm (18792 cm�1) by the cation is only possible for

the higher 2E1/2 state. This absorption will excite [CH3I]þ(X)

to higher vibrational levels of the ionic A state. The energy

required for the 2E1/2 state of [CH3I]þ to dissociate into

CH3þ Iþ on the A state potential surface is 2.7 eV

(21777 cm�1). Thus, following single-photon absorption at

532 nm (hn¼ 2.33 eV), ionic dissociation is energetically not

allowed on the A state surface. However, it has been shown

that the A state predissociates via the X state. Thus, for one-

photon excitation of [CH3I]þ at 532 nm, the only energetically

allowed dissociation reaction is formation of [CH3]þ ions:

CH3IþðX2E1=2Þ ! CHþ3 þ I ð2P3=2Þ; �E ¼ 48:5 kcal mol�1

ð1Þ

! CHþ3 þ I ð2P1=2Þ; �E ¼ 70:2 kcal mol�1 ð2Þ

However, along with [CH3]þ, [I]þ ions are observed down

to the lowest intensity (�3� 109 W/cm2) used in the present

work. It has been observed that [CH3I]þ ions produced in the

higher vibrational levels of the A state, either by direct

autoionization or following conversion from the B state,

dissociate into CH3þ Iþ.22 However, internal conversion (IC)

from the ionic A state to the X state, followed by dissociation

on the ground state surface giving rise to CH3þþ I, is also

competing simultaneously as shown below:

CH3Iþð~AAÞ ! CH3 þ Iþ ðdirect dissociationÞ ð3Þ

CH3Iþð~AAÞ �!IC CH3IþðXÞ ! CHþ3 þ I ð4Þ

Presence of [I]þdown to the 3� 109 W/cm2 intensity shows

that two or more photons are absorbed by the [CH3I]þ so that

direct dissociation on the A ([CH3I]þ) state becomes

competitive with the internal conversion to the X ([CH3I]þ)

state. Under low intensity (�3� 109 W/cm2), the integrated

ion signals of [I]þ and [CH3]þ are comparable. With

increasing intensity, the [I]þ signal increases and then a

nearly constant ratio of [I]þ/[CH3]þ is maintained over the

entire intensity region. These two channels (3 and 4),

producing [I]þ and [CH3]þ, respectively, have been shown

to be statistical and in competition with one another.22,23 As a

result, both [I]þ and [CH3]þ are observed in most MPI studies

on CH3I. The nearly constant ratio of [I]þ/[CH3]þ over a wide

intensity range suggests that the rate of dissociation on the A

state surface and internal conversion to the X state are nearly

the same.

Mass spectra and power dependence at 355nmThe A state absorption for gas-phase CH3I lies in the region

330–210 nm, and thus 355 nm radiation also lies outside the

single-photon absorption range;3 355 nm is a non-resonant

excitation wavelength for CH3 as well as I atoms. In the

case of 355 nm photoionization, at low intensity of the

pump laser (�1� 109 W/cm2) only [CH3]þ, [I]þ and

[CH3I]þ ions were observed (Fig. 2(c)). Under high intensity

conditions (�2� 1010 W/cm2), in addition to the above ions,

[CI]þ and [CH2I]þ ions were also observed in very low yield.

For [CH3I]þ, [I]þ and [CH3]þ, the ln-ln plots of laser energy

vs. ion signal are shown in Fig. 3(b). A laser power

dependence of�2 was observed for all the ions ([CH3]þ¼ 1.8,

1.8, [I]þ¼ 1.7 and [CH3I]þ¼ 1.7). These results indicate the

intermediacy of a two-photon populated state at �7 eV

during the ionization process, similar to the 532 nm results. It

has been shown that the Rydberg C state lying at �7 eV is

resonantly populated by two-photon absorption at 355 nm

(Fig. 4).13 From the C state the molecules can either undergo

predissociation via the A state into CH3þ I (2P1/2 or 2P3/2), or

absorb another photon and be ionized to form the precursor

ion [CH3I]þ. Since the geometries of neutral and ionic CH3I

are nearly the same, the single-photon-induced up-pumping

step from the Rydberg C state of CH3I to ionization levels is

Franck-Condon favored, and this (2þ 1) resonance-enhanced

multiphoton ionization process (REMPI) will compete with

predissociation via the A state.

Appearance potentials for [CH3]þ and [I]þ from CH3I(X)

are 12.28 and 12.98 eV, respectively.23 Thus, absorption of

three photons (total energy 10.47 eV) would give rise to

formation of [CH3I]þ (2E3/2 and 2E1/2) only.24,25 However,

[CH3]þ and [I]þ are always detected down to the lowest

energies. As mentioned above, the A!X electronic absorp-

tion of [CH3I]þ lies in the visible region with origin at 14

359 cm�1 for 2E1/2 and at 19 409 cm�1 for 2E3/2. Dissociation of

[CH3I]þ into CH3þþ I on the X state is located at 16 973 and

22 022 cm�1 for the 2E1/2 and 2E3/2 states, respectively.24

Absorption of a 355 nm photon (28 190 cm�1) by [CH3I]þwill

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

0

1

2

3

-0.5 0.0 0.5 1.0 1.5 2.00

1

2

3

4

5 I+

CH3

+

CH3I+

ln(S

igna

lInt

ensi

ty)

ln (laser energy)

Power Dependence at 355 nm

Power Dependence at 532 nm

ln(S

igna

lInt

ensi

ty)

ln (laser energy)

I+

CH3

+

CH3I+

(a)

(b)

Figure 3. ln-ln plots of laser intensities and signal intensity.

(a) At l¼ 532nm. CH3þ (&) n¼ 2.3, Iþ(*) n¼ 2.7 and CH3I

þ

(~) n¼ 2.3; (b) At l¼ 355nm. CH3þ(&) n¼ 1.8, Iþ(*) n¼ 1.7

and CH3Iþ(~) n¼ 1.7.



excite the parent ion to the higher vibronic levels of the A

state which can undergo dissociation on either the A or the X

surfaces. The corresponding dissociation energies are given

below.

CH3IþðX2E1=2Þ ! CHþ3 þ I ð2P3=2Þ;�E ¼ 48:5 kcal mol�1

ð5Þ

CH3IþðX2E1=2Þ ! CH3Iþð~AAÞ ! CH3 þ Iþ ð3P2Þ;�E ¼ 62:4 kcal mol�1

ð6Þ

Similarly for the X (2E3/2) state of [CH3I]þ, the dissociation

energies are:

CH3IþðX2E3=2Þ ! CHþ3 þ I ð2P3=2Þ;�E ¼ 63:0 kcal mol�1 ð7Þ

CH3IþðX2E3=2Þ ! CH3Iþð~AAÞ ! CH3 þ Iþð3P2Þ;�E ¼ 76:8 kcal mol�1 ð8Þ

All these reactions are energetically allowed since the

energy of one 355 nm photon is higher than that required by

channels 5–8.

In earlier studies on CH3I it was reported that, if [CH3I]þ is

prepared in the higher vibrational levels of the A state, the ion

can directly dissociate into CH3þ Iþ, or alternatively can

undergo internal conversion and dissociate exclusively on

the ground-state potential energy surface giving CH3þþ I.23

In the present work both the fragment ions, i.e. [I]þ and

[CH3]þ, are observed along with the parent ion, in agreement

with the previously reported results.22,26 The observation of

both these ions with the same power dependence in our work

further supports the ionic dissociation in both A and X states.

The lifetime of the ionic A state has been estimated to be of the

order of 10�10 s,25 which is sufficient for absorption of

additional photons with increasing intensity. These addi-

tional photons could excite the ion to higher energy states,

which opens up the possibility of other dissociation channels

(see below).

Multiphoton excitation at 532nm: intermediateA state vs. direct excitationIn our present studies with nanosecond pulses of 532 nm, the

molecule reaches vibronic levels of the A state (Fig. 4) upon

two-photon excitation. Thus it would be interesting to com-

pare the photophysical/photochemical processes occurring

at 532 nm photoionization with that of its frequency-doubled

output, i.e. 266 nm. In view of the dissociative nature of the A

state, CH3I should undergo fast dissociation leading to

absence of observable molecular ions at both these wave-

lengths. As expected, at 266 nm, the parent ion was found

to be absent in our mass spectral experiments (Fig. 2(b)), simi-

lar to the ns excitation results reported previously.14,22 How-

ever, using picosecond excitation, Szaflarski and El-Sayed13

were able to observe parent ions. This difference between

nanosecond and picosecond studies is due to the fact that

resonant single-photon absorption in the A state leads to

ultrafast dissociation of CH3I on the time scale of �100 fs.

Thus, higher pumping intensities are required for the forma-

tion of molecular ions via an intermediate A state, and this

could be achieved in picosecond experiments and not with

nanosecond excitation. However, at 532 nm, the parent ion

is observed along with the fragment ions. At 532 nm, a power

dependency of 3 was observed which suggests that three-

photon absorption to the C state occurs at 532 nm. From the

power dependence studies and observation of parent ions

in the multiphoton ionization regime (even at low laser

intensity, �3� 109 W/cm2), we conclude that excitation to

vibronic levels in the C state occurs by a simultaneous

three-photon transition. It is possible that the A state may

act as a virtual intermediate level.

At 266 nm, the CH3I mass spectrum shows a large peak for

[I]þ. This is due to efficient (2þ 1) REMPI of photogenerated

I(2P3/2) atoms via a (7P2D5/2 2P3/2) transition. Due to this

higher sensitivity, it might be possible that we may not have

been able to detect minor amounts of the parent ion formed

by a ladder mechanism using our nanosecond laser. So, to

further confirm our findings, studies on CH3I were also

conducted using wavelengths around 564 nm and 282 nm

(near the maxima of rhodamine 6G output). I(2P1/2) is

reported to have several (2þ 1) REMPI transitions in the

275–285 nm region, so, in the first set of experiments, a (2þ 1)

REMPI spectrum for iodine was recorded in the 281.4–

282.9 nm region (Fig. 5(b), inset). In this wavelength region

we observed two resonances, both of which could be ascribed

to iodine 2P1/2. An off-resonance wavelength of 282.1 nm was

then selected to observe the effect of A state mediated

98952 cm-1 103800 cm-1

C-state

B-state

A-state

96338 cm-1

81979 cm-1 76930 cm-1

8s

CH3I (X)

CH3 + I+

CH3++ I

B

A

2E1/2

IC

CH3I+

2E3/2

2E1/2

2E3/2

355 nm

532 nm

Ene

rgy

(x 1

03 cm

-1)

10

20

30

40

50

60

70

80

90

100

110

130

120

X

X

dissociation into CH3 + I followed by their ionization

282.1 nm

266 nm

Figure 4. Energy level diagram of neutral CH3I and its

cation. The values given in cm�1 are taken fromWalter et al.24



ionization. As shown in Fig. 5(b), there was no evidence of

parent ion formation at this wavelength, in agreement with

the 266 nm results. In contrast, at 564.2 nm, the parent ion was

detected (Fig. 5(a)), similar to the results using 532 nm. These

results suggest that, at 266 and 282.1 nm, a ladder-switching

process (dissociation followed by ionization) is the major

pathway, while, at 532 and 564.2 nm, a different ladder-

climbing process (ionization followed by dissociation) is

more dominant.

[CH2I]þ and [CI]þ

Formation of [CH2I]þ and [CI]þ ions has not been reported in

earlier MPI studies using nanosecond lasers. In the present

work, low yields of [CI]þ and [CH2I]þ ions were observed

under high laser intensity (�2� 1010 W/cm2) at 355 nm.

[CH2I]þ can be formed as a result of MPI of neutral CH2I

arising from cleavage of a H–CH2I bond. However, this

channel has not been reported in C state photoexcitation

studies. This is due to the fact that the C–H bond energy is

rather high compared to C–I bond energy, and exclusive

dissociation of the C–I bond takes place on photoexcitation.

The experimentally measured bond-dissociation energies

are:27,28

CH3I! CH3 þ Ið2P3=2Þ; �E ¼ 55:6 kcal mol�1 ð9Þ

! CH3 þ Ið2P1=2Þ; �E ¼ 77:3 kcal mol�1 ð10Þ

! Hþ CH2I; �E ¼ 101 kcal mol�1 ð11Þ

Indirect evidence for CH2I formation was provided in a

vacuum UV (121.6 nm) photodissociation study of CH3I, in

which fast H atoms produced by direct dissociation of CH3I

into HþCH2I were detected.28

In a photoelectron-photoion coincidence (PEPICO) study,

formation of [CH2I]þ was detected from the B state of the

parent ions,29 which is located at an energy of 13.9 eV.

Although the channel CH2IþþH is energetically accessible at

12.74 eV, [CH2I]þ is detected only above 13.9 eV.30 This has

been attributed to the fact that on the B state there is a

competition between dissociation into HþCH2Iþ and rapid

internal conversion leading to slower dissociation over the

A/X states resulting in the formation of IþþCH3 and/or

CH3þþ I. As mentioned earlier, the 355 nm photon can excite

the CH3Iþ(X) ions to the higher vibrational levels of the A

state or the B state (13.90 eV). At lower intensities

(�1� 109 W/cm2) only [CH3]þ, [I]þ and [CH3I]þ are

observed, showing that higher levels of the ionic B state are

not populated. However, at higher intensity (�2� 1010 W/

cm2), more photons are absorbed and the internal energy of

[CH3I]þ ions is much higher. Although formation of [I]þ and

[CH3]þ are still major channels, some of the ions do dissociate

directly on the B state giving rise to [CH2I]þ. These results

show that at higher intensities direct dissociation starts

competing with dissociation on A/X state surfaces.

In our studies, [CI]þ could be produced from dissociation

of the parent ions or from dissociation of the fragment ion

[CH2I]þ. Theoretical calculations were performed to obtain

the reaction energies of different channels for [CH3I]þ

dissociation. In addition to the two most abundant product

channels giving [CH3]þ and [I]þ (channels 5–8), two other

channels forming [CHI]þ and [CI]þ were studied:

CH3Iþ ! CHIþ þH2; �E ¼ 92:6 kcal mol�1 ð12Þ

! Clþ þH2 þH; �E ¼ 165:4 kcal mol�1 ð13ÞBoth of these channels involve molecular elimination of H2,

which has additional barriers of a few kcal mol�1. These

results show that both [CHI]þ and [CI]þ can be formed from

[CH3I]þ. However, formation of [CI]þ (channel 13) involves a

three-body dissociation and requires additional energy of

73 kcal mol�1 over channel 12. Thus, if [CI]þ is produced from

the dissociation of the precursor ion, [CHI]þ should also be

detected due to its lower reaction energy. These conclusions

are supported by the VUV photoionization studies on CH3I

where the appearance potential for [CHI]þ (14.5 eV) was

reported to be lower than that of [CI]þ (19.5 eV).30 This

implies that, under high laser intensity (�2� 1010 W/cm2), as

for the conditions of our work, both [CHI]þ and [CI]þ should

be detected. However, no [CHI]þ signal was observed. Thus,

we conclude that [CI]þ is not produced by direct dissociation

of [CH3I]þ.

The other possible way for [CI]þ to be formed is by

photodissociation of [CH2I]þ ions. We calculated the energy

required for dissociation of [CH2I]þ into different possible

channels, and the following values were obtained:

CH2Iþ ! Clþ þH2; �E ¼ 91:4 kcal mol�1 ð14Þ

! CHIþ þH; �E ¼ 123:0 kcal mol�1 ð15Þ

20 40 60 80 100 120 140 160 180 200

0.0000

0.0001

0.0002

0.0003

0.0004

0.0005

20 40 60 80 100 120 140 160 180 200

0.00

0.01

0.02

0.03

0.04

0.05

CH3I at 564.2 nm, 5 mJ (a)

Inte

nsity

(arb

.uni

ts)

281.5 282.0 282.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

I+si

gnal

(arb

.inte

nsi

ty)

Wavelength (nm)

CH3I at 282.1 nm, 5 mJ

Inte

nsity

(arb

.uni

ts)

m/z

(b)

Figure 5. MPI mass spectra of CH3I on excitation (a) at

564.2 nm at an energy of 5mJ/pulse, and (b) at 282.1 nm at

an energy of 5mJ/pulse. The inset shows (2þ 1) REMPI

spectra of iodine atoms generated from photodissociation of

CH3I in the 281.4–282.9 nm region at an energy of �1mJ/

pulse.



! CH2 þ Iþ; �E ¼ 121:3 kcal mol�1 ð16Þ

! CHþ2 þ I; �E ¼ 121:7 kcal mol�1 ð17Þ

For channel 14, a barrier height (DVf) of 101.3 kcal mol�1

was calculated. The absence of [CHI]þ shows that, even

though having a dissociation barrier, the molecular elimina-

tion of H2 is effectively competing with the C–H and C–I

bond dissociations.

In the case of 532 nm photoionization, only [CI]þ was

observed. There is a possibility that, at 532 nm also, [CI]þ is

generated from dissociation of the parent ion [CH3I]þ or of

the [CH2I]þ fragment, as observed in the case of 355 nm.

However, the absence in the TOFMS spectra of [CH2I]þ and

[CHI]þ, which have lower appearance potentials than CIþ,

suggests that formation of [CI]þ can be explained by three-

body dissociation of neutral CH3I; theoretical calculations

gave a value of 201 kcal mol�1 for this process:

CH3I! ClþH2 þH; �E ¼ 201 kcal mol�1 ð18Þ

This three-body dissociation channel is energetically not

allowed from the C state which has a total energy of 7 eV

(161.4 kcal mol�1). As shown in Fig. 1, absorption of the

fourth 532 nm photon excites CH3I from the Rydberg C state

to an upper state with a total energy of 9.32 eV; this is a mixed

state having valence and 8s Rydberg state characteristics and

has a total energy of 215.1 kcal mol�1, which makes this three-

body dissociation channel energetically feasible. The neutral

CI so produced would then undergo ionization (IP¼ 8.17 eV)

by absorbing additional photons from the same laser pulse.

Our arguments are supported by the VUV study of CH3I at

10.2 eV, in which fast as well as slow H atoms were detected

using a high-n-Rydberg time-of-flight (HRTOF) technique.28

The fast H atoms were ascribed to the direct dissociation into

HþCH2I and the slow H atoms were due to the CIþH2þH

channel. This channel is not expected to be significant in the

case of 355 nm, since the absorption of a third photon from the

C state drives the molecule directly into the ionization

continuum.

CONCLUSIONS

Photoionization of CH3I has been studied at 532 and 355 nm

using a time-of-flight mass spectrometer. The studies show

that a resonant intermediate level is reached after CH3I

absorbs three and two photons respectively at these wavele-

ngths. The resonant vibronic level is identified as 6s (2E1/2,

2v2, v2þv3). Along with the precursor ion, [CH3]þ and [I]þ

are always observed down to the lowest intensities. In the

case of 355 nm photoionization the fragment ions [CI]þ and

[CH2I]þ have been observed for the first time using a nanose-

cond laser; the former is formed as a result of the direct dis-

sociation on the B state of the [CH3I]þ ion, whereas the latter is

produced as a result of [CH2I]þ dissociation. In the case of

532 nm photoionization, [CI]þ formation has been explained

based on the three-body dissociation into CIþ H2þH from

the 9.32 eV excited state followed by its ionization. Studies at

564.2 nm were found to be in agreement with those con-

ducted at 532 nm, suggesting the dominance of ladder-climb-

ing processes at these wavelengths. Studies conducted at 266

and 282.1 nm, which access the A state directly via one-

photon transitions, showed a fast ladder-switching mechan-

ism as the major process.

AcknowledgementsThe authors are thankful to Prof. N.J. Mason, The Open

University, Milton Keynes, UK, for a preprint of his work

on CH3I, and to a referee for very thoughtful suggestions.

REFERENCES

1. Mulliken RS. Phys. Rev. 1935; 47: 413.2. Gedanken A, Robin MB, Yafet Y. J. Chem. Phys. 1982; 76:

4798.3. Fahr A, Nayak AK, Kurylo MJ. Chem. Phys. 1995; 197: 195.4. Dobber MR, Buma WJ, De Lange CA. J. Chem. Phys. 1993; 99:

836.5. Syage JA. Chem. Phys. Lett. 1993; 212: 124.6. Wang PG, Ziegler LD. J. Chem. Phys. 1991; 95: 288.7. Guo H. Chem. Phys. Lett. 1992; 193: 527.8. Janssen MHM, Dantus M, Guo H, Zewail AH. Chem. Phys.

Lett. 1993; 214: 281.9. Khundkar LR, Zewail AH. Chem.Phys.Lett. 1987; 142: 426.

10. Graham P, Ledingham KWD, Singhal RP, Hankin SM,McCanny T, Fang X, Kosmidis C, Tzallas P, Taday PF,Langley AJ. J. Phys. B: At. Mol. Opt. Phys. 2001; 34: 4015.

11. Lehr L, Weinkauf R, Schlag EW. Int. J. Mass Spectrom. 2001;206: 191.

12. Siozos P, Kaziannis S, Kosmidis C. Int. J. Mass Spectrom.2003; 225: 249.

13. Szaflarski DM, El-Sayed MA. J. Phys. Chem. 1988; 92: 2234.14. Choi YK, Koo YM, Jung KW. J. Photochem. Photobiol A: Chem.

1999; 127: 1.15. Sharma P, Vatsa RK, Maity DK, Kulshreshtha SK. Rapid

Commun. Mass Spectrom. 2004; 18: 2383.16. Sharma P, Vatsa RK, Maity DK, Kulshreshtha SK. Chem.

Phys. Lett. 2003; 382: 637.17. Rao PMR, Das NC, Sekhar BNR, Padmanabhan S, Shastri A,

Bhattacharya SS, Roy AP. Nucl. Instrum. Methods Phys. Res.2001; 467/468: 613.

18. Dunning TH Jr. J. Chem. Phys. 1989; 90: 1007.19. Martin JML, Sundermann A. J. Chem. Phys. 2001; 114: 3408.20. Bergner A, Dolg M, Kuechle W, Stoll H, Preuss H. Mol. Phys.

1993; 80: 1431.21. Schmidt MW, BalDridge KK, Boatz JA, Elbert ST, Gordon

MS, Jensen JH, Koseki S, Matsunaga N, Nguyen KA, Su SJ,Windus TL, Dupuis M, Montgomery JA Jr. J. Comput. Chem.1993; 14: 1347.

22. Zhang B, Wang X, Lou N, Zhang B, Wei J. Spectrochim. ActaPart A 2001; 57: 1759.

23. Powis I. Chem. Phys. 1983; 74: 421.24. Walter K, Weinkauf R, Boesl U, Schlag EW. J. Chem. Phys.

1988; 89: 1914.25. Kawasaki M, Sato H. J. Chem. Phys. 1987; 87: 5739.26. Chupka WA, Colson SD, Seaver MS, Woodward AM. Chem.

Phys. Lett. 1983; 95: 171.27. Eppink ATJB, Parker DH. J. Chem. Phys. 1999; 110: 832.28. Amaral G, Xu K, Zhang J. J. Phys. Chem. A 2001; 105: 1115.29. Eland JHD, Frey R, Kuestler A, Schulte H, Brehm B. Int. J.

Mass Spectrom. Ion Phys. 1976; 22: 155.30. Onley TN, Cooper G, Brion CE. Chem. Phys. 1998; 232: 211.



photoionization of ch3i mediated by the c state in the visible and ultraviolet regions

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