ch br vmi-rempi studies in combination with mass resolved ... ·...

1

CH3Br VMI-REMPI studies in combination with Mass resolved REMPI;

Summary report (July, 2017) for experimental data from FORTH and analysis

from 2016.

NB: processed data and data analysis are to be found under

https://notendur.hi.is/~agust/rannsoknir/Crete/Crete-1.htm

Content list:

Topics: pages:

I. Mass Resolved (MR)-REMPI and VMI-REMPI….. 2 – 15

1) MR-REMPI……………………………………. 2 - 3

2) VMI-REMPI; one-color (CHn+, Br+, CBr+)….. 4 – 13

3) VMI-REMPI; one-color (e- detection)………… 14-18

4) VMI-REMPI; two-color (Br/Br* detection)…… 19-23

5) VMI-REMPI; two-color (CH3 detection)……. 24

Updated: 170809 / IN PROGRESS

https://notendur.hi.is/~agust/rannsoknir/Crete/Crete-1.htm

2

I. Mass Resolved REMPI (MR-REMPI) and VMI-REMPI

1) MR-REMPI:

Results:

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160904-CH3Br(1).pptx

slides 7, 14-16

(see also: https://notendur.hi.is/~agust/rannsoknir/papers/jpcA114-9991-10.pdf ;

Fig. 1a):

Mass spectra and corresponding REMPI spectra have been recorded for the one-

color excitation corresponding to two-photon resonance excitation within the range

of 66000 – 80000 cm-1:

Fig 1a: Mass spectra vs. CH3

+-REMPI spectra. The mass spectra are normalized to

a common intensity for the CH3+ peaks.

Conclusions:

Strongest mass signals are observed for the CH3+ ion

(see Fig. 1a above and slides 15-16 in

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160904-CH3Br(1).pptx )

Relative signal contributions of other ions (I(M+)/I(CH3+)); M = CH2,CH, Br)

increases as the 2hv scale approaches the CH3+ + Br- threshold

(see Fig. 1a above and https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160904-

CH3Br(1).pptx

slides 15-16)

82

80

78

76

74

72

70

68

66

x10

3

13 15 79 91

iBr+ (i=79,81) CBr+ (i=79,81)

CHn+

n=1,2,3

Intensity Mw / amu

E / cm-1

CH3+ + Br-

x 2.5


https://notendur.hi.is/~agust/rannsoknir/papers/jpcA114-9991-10.pdf




3

The CH3+-REMPI spectrum can be divided into two major contributions, depending

on the ion formation mechanisms (this is further supported by one-color VMI_REMPI

data below), i.e. i) „an underlying „continuum“ contribution“, gradually increasing

with 2h, which is due to one-photon photodissociation process (1hv excitation to

repulsive valence states) followed by three-photon (3hv) nonresonance ionization of

CH3 (marked as 1h) AND ii) „Rydberg state spectra contributions“ on top of (i),

which is due to initial two-photon resonance excitations to Rydberg states (marked as

2hr):

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160908-CH3Br(2).pptx; slide 52:

Fig. 1b

see also https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160904-CH3Br(1).pptx

slide 5 (top)

8000078000760007400072000700006800066000

2h / [cm-1

]

[3/2]np;w

[1/2]np;w

[3/2]nd;w

[1/2]nd;w

5 6 7

0 0 02

n =

w =

5 6

02

02

0

02

02

02

02

4 6

4 5

3 2 1 3 2

3 2

3 2 1

3 2

66

01

9

68

68

4

68

46

1

72

65

5

72

97

7

75

41

87

56

86

75

90

5

78

22

5

78

37

0

78

19

3

78

40

1

79

61

0

80

64

0

80

75

8

80

67

4

80

88

12hvr:signals followingtwo-photonresonance excitation

1hv:Signals followingone-photon excitation



4

2) Results:

VMI-REMPI data were collected for one color excitation corresponding to two-

photon excitation to number of Rydberg states within the range of 66000 – 80000 cm-

1, for twelve wavenumber values in total:

https://notendur.hi.is/~agust/rannsoknir/Crete16/XLS-160912.xlsx; sheet: „Waves“:

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160904-CH3Br(1).pptx ; slide 17:

Fig. 2a

Images for CHn+; n = 3-0, Br+ and CBr+ ions were detected and recorded to a different

amount:


„Wave number“ entries in the table , below, represent measurements which were

performed (red shaded areas represent „nonexisting measurements“):

8000078000760007400072000700006800066000

2h / [cm-1

]

[3/2]np;w

[1/2]np;w

[3/2]nd;w

[1/2]nd;w

5 6 7

0 0 02

n =

w =

5 6

02

02

0

02

02

02

02

4 6

4 5

3 2 1 3 2

3 2

3 2 1

3 2

66

01

9

68

68

4

68

46

1

72

65

5

72

97

7

75

41

87

56

86

75

90

5

78

22

5

78

37

0

78

19

3

78

40

1

79

61

0

80

64

0

80

75

8

80

67

4

80

88

1

66

50

3

67

27

5

68

88

2

69

94

7

74

24

9

76

68

97

71

65

https://notendur.hi.is/~agust/rannsoknir/Crete16/XLS-160912.xlsx







5

KER spectra and angular distributions were derived from the images:

KER spectra:

See processed data: https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160908-

CH3Br(2).pptx; slides 2 - 64

See assignments/KER spectra predictions:

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160922-CH3Br(2).pptx;

Angular distributions: https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160908-

CH3Br(2).pptx; slides 65-109

Conclusions concerning the strongest ion signals (i.e. CH3+):

CH3+ KERs (use xx KERs):


Fig. 2b

10

8

6

4

2

0

2.01.51.00.50.0

CH3+

eV

CH3++Br-

Ry(2hv/cm-1):

79610 (16.09.16)78370(12.9.16)77165(15.9.16)75905(14.9.16)74249815.9.16)72973(7.9.16)68684(14.9.16)67275(13.9.16))66503(8.9.16)

66019 (6.9.16)







6

Some of the KERs show two major contributions according to the two different ion

formation mechanisms (i) and (ii) (see (1); conclusion above) (ii) broad „underlying“

continuum feature corresponding to 2hr (ii) and (i) sharp peaks on top :


Fig 2c

2hr (ii) spectral contributions:

a) The lowest energy excitation KERs (2h = 66019– 68684 cm-1) show vibrational

structure in the 2hr spectra (ii). This is due to the following CH3+ ion formation:


Fig. 2d

i.e. two-photon resonance excitation to a Rydberg states (2hr) followed by one-

photon photodissociation via a superexcited state (1hpd) followed by one-photon

3.0

2.5

2.0

1.5

1.0

0.5

0.0

2.52.01.51.00.50.0

2hvr:signals followingtwo-photonresonance excitation

1hv:Signals followingone-photon excitation

77165 (15.09.16)

eV

Example of a1hv and 1hvr

signal contributions in a CH3 KER spectrum:

CH3Br

CH3Br** (Ry)

CH3**(v´+n) + Br/Br*:CH3**(v´) + Br/Br*

CH3+ + Br/Br*

Vibrational ladder for The OPLA vibrationalmode

Molecularexcitation

Dissoci-ation

Fragmentionization

KER

Intensity

2hvr

1hvpd

1hvi



7

ionization (1hi) of CH3**(3p2A2): (2hr+1hpd +1hi) REMPI. NB: The mechanism

involves three-photon excitation (3h = 2hr+1hpd ) prior to the dissociation.

b) The higher energy excitation KERs (2h > 68684 cm-1) involve analogous ion

formation mechanisms to that described in (a) including CH3** fragments of higher

energy than CH3**(3p2A2) as well.

(see: https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160908-CH3Br(2).pptx;

slide 41 for the energetics AND https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-

160904-CH3Br(1).pptx ; slide 28 for CH3** states)

NB: This is further confirmed by PES (see below).

Since the two spectral contributions (i) and (ii) involve one-photon (1h) and three-

photon (3h) excitations prior to dissociation it is convenient to compare the KERs on

two different h scales, (i) where KERs, for excitation frequencies i, have been

shifted by h = h = h(0 – i) with respect to a reference spectrum (excitation

frequency 0) AND (ii) where the KERs have been shifted by 3h = 3h = 3h(0 –

i). This corresponds to a normalization of the spectra of same formation mechanism

((i) or (ii)) with respect to energy thresholds of fragment species.

(analogous comparison is performed in references

https://notendur.hi.is/~agust/rannsoknir/papers/jcp130-034304-09.pdf

(Fig6) and https://notendur.hi.is/~agust/rannsoknir/papers/pccp11-2234-09.pdf

(Fig.6)):

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160908-CH3Br(2).pptx;slide 53:

Thus:

Fig. 2e


Thus, for (ii):

Threshold (2)

Threshold (1)

nh

vex

cita

tio

ns

n =

1,2

,3,…

KER spectra

3

h

0

Relative intensity




https://notendur.hi.is/~agust/rannsoknir/papers/jcp130-034304-09.pdf

https://notendur.hi.is/~agust/rannsoknir/papers/pccp11-2234-09.pdf



8

Fig. 2f

1h (i) spectral contributions:

The highest energy excitation KERs (2h = 77165– 79610 cm-1) show sharp peaks,

particularely so for 2h = 77165 and 79610 cm-1, for which the 1h(i) ion formation

contribution is the major (2hvr(ii) minor) as can be seen from the CH3+ REMPI

spectrum (see Fig 2a above) These are due to the following ion formation:


Fig. 2g

15

1050

43

21

03

h

/eV

CH3(3p2A2)+Br

CH3(3d2E)+Br

CH3(3d2A1)+Br

CH3(4p2A2)+Br

CH3(4f2E)+Br

3h

v ex

cita

tio

ns

Thresholds

CH3(3p2A2)+Br*

CH3(3d2E)+Br*

CH3(3d2A1)+Br*

CH3(4p2A2)+Br*

CH3(4f2E)+Br*

79610 (16.09.16)78370(12.9.16)77165(15.9.16)75905(14.9.16)74249815.9.16)

72973(7.9.16)68684(14.9.16)67275(13.9.16))66503(8.9.16)66019 (6.9.16)

120

100

80

60

40

20

0

x1

03

16141210864

CH3Br

CH3 + Br/Br*

CH3

+ + Br

-

CH3Br+/CH3Br

+*

CH3** + Br/Br*

CH3

+ + Br/Br*

r/A

1h

3hi


9

i.e. one-photon photodissociation (1hph) to form CH3(v1,v2,v3,v4) + Br/Br* followed by

three-photon (two-photon for 2h = 79610 cm-1;NB) nonresonance ionization (3hi (2hi)) of

CH3(v1,v2,v3,v4): (1hpd + 3hi(2hi)) REMPI:


Fig. 2h

Vibrational analysis of the lowest energy excitation KERs (2h = 66019, 66503,

67275, 68684 cm-1) revealed vibrational spectroscopic parameters for the CH3** (3p 2A2) Rydberg state, vibrational mode 2 / OPLA:

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160908-CH3Br(2).pptx;slides

58, 65-68:

Fig, 2i

15

1050

2.5

2.0

1.5

1.0

0.5

0.0

h

/e

V

CH3(0000)+Br(1.5856)

CH3(0100)+Br(1.5224)(1.4394)CH3(0001)+Br

(1.2725)CH3(1000)+Br

(1.2562)CH3(0010)+Br

Thresholds

CH3(0000)+Br*(1.2016)

CH3(0100)+Br*(1.1384)CH3(0001)+Br*(1.0555)

CH3(1000)+Br*(0.8885)

CH3(0010)+Br*(0.8722)

79610 (16.09.16)78370(12.9.16)77165(15.9.16)75905(14.9.16)74249815.9.16)

72973(7.9.16)68684(14.9.16)67275(13.9.16))66503(8.9.16)66019 (6.9.16)

1h

v ex

cita

tio

ns

Hot bands, i.e. due toVibrationall „hot“ CH3Br#

D0 stands for CH3Br -> CH3 + Br

66503 cm-1

eV



10

The we parameter (1310 cm-1) is to be compared with corresponding values reported in

NIST(http://webbook.nist.gov/ ) for CH3**(3p 2A2), CH3+(X) of 1323 and 1359 +/- 7 cm-1

respectively. we for CH3(X) on the other hand is significantly lower (606 cm-1;

http://webbook.nist.gov/)

(see also: https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160904-CH3Br(1).pptx slides

27, 28, 45)

Conclusions concerning the strongest ion signals (i.e. CH3+):

CH3+ angular distributions (xx signals):


The angular distributions of the CH3+ images, corresponding to the 2hr (ii) ion

formation mechanisms, display shapes corresponding to parallel to “neutral” (i.e.

equal parallel and perpendicular character) transitions.


Anisotropy parameters ( 2) extracted from the angular distributions as a function of

the two-photon excitation wavenumber are in the range of about 0 to 2.

a) Medium values of about 0.7 – 2.0 are derived for excitations to the four lowest

states (2h = 66019, 66503, 67275, 68684 cm-1), which involve a dominant ionization

of the CH3**(3p 2A2) Rydberg state following two-photon resonance excitation (2hr)

to CH3Br (5p) Rydberg states and a further one-photon photodissociation (1hpd) step.

b) A large 2 value of about 2.0, corresponding to a purely parallel transition, is

derived for excitation to the 2h = 72911 cm-1 state which involves a dominant

ionization of the CH3**(3d 2E) and/or CH3**(3d 2A1) Rydberg states (see (3) below)

following two-photon resonance excitation (2hr) to CH3Br (4d) Rydberg states and a

further one-photon photodissociation (1hpd) step.

c) Close to zero vales of 2, corresponding to “neutral” transition are derived for

excitations to the states 2h = 75905 and 79610 cm-1 state, which involve two-photon

resonance excitation (2hr) to CH3Br (6p) and (7p) Rydberg states.

Fig. 2j

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

78767472706866

x103

2

1.2932 660190.71411 665031.1593 672750.71876 686842.0226 729771.3393 74249-0.058 759050.5568 771650.2957 783700.0092239 79610

CH3+

http://webbook.nist.gov/

http://webbook.nist.gov/




11

The sharp peaks due to the 1h(i) ion formation mechanism (see Fig. 2h) which

appear near h = 1.5 eV are due to formation of CH3(bending modes, n2,n4) + Br

(ground state) after 1hpd. Angular distributions of the CH3+ fragments are

perpendicular () in nature with 2 = -0.15 and -0.24 for 2h = 77165 cm-1 and 79610

cm-1, respectively:

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160908-CH3Br(2).pptx; slide 79,80:

Fig. 2k

Fig. 2l

This suggests that the major 1hpd transition corresponds to excitation of CH3Br to the

repulsive state 3Q1 and/or to the 1Q1 state (both of which are perpendicular transitions)

followed by a dissociation on the diabatic curves to form CH3(bending modes) + Br:

https://notendur.hi.is/agust/rannsoknir/papers/pccp1-747-00.pdf

3.5

3.0

2.5

2.0

1.5

1.0

0.5

150100500

3.0

2.5

2.0

1.5

1.0

0.5

0.0

2.52.01.51.00.50.0 eV

q

2 = 0.5568

2 = 0.89093

2 = -0.14974

2 = 1.0978

2 = 1.5678

Ry(2hv/cm-1):77165(15.9.16)

XZ

CH3+

3.5

3.0

2.5

2.0

1.5

1.0

150100500

8

6

4

2

0

2.52.01.51.00.50.0eV

q

2 = 0.0092239

2 = 1.8876

2 = 1.4666

2 = -0.23585

2 = 1.4741

Ry(2hv/cm-1):79610(16.9.16)

XZ

CH3+



12


appear near h = 1.26 eV are due to formation of CH3(stretching modes, 1,3) + Br

(ground state) after 1hpd. Angular distributions of the CH3+ fragments are parallel ()

in nature (see Figs, 2k and 2l above) with 2 = +0.89 and +1.47 for 2h = 77165 cm-1

and 79610 cm-1, respectively. This suggests that the major 1hpd transition

corresponds to excitation of CH3Br to the repulsive state 3Q0 followed by a

dissociation on the adiabatic curves to form CH3(stretch modes) + Br:



appear near h = 1.14 eV are due to formation of CH3(bending modes, 2) + Br* (SO

excited state) after 1hpd. Angular distributions of the CH3+ fragments are almost

purely parallel () in nature (see Fig. 2l above) with 2 = +1..89 for 2h = 79610 cm-1.

This suggests that the major 1hpd transition corresponds to excitation of CH3Br to the

repulsive state 3Q0 followed by a dissociation on the diabatic curves to form

CH3(bending mode) + Br*:


Conclusions concerning the Br+ ion signals

Br+ KERs (use xx KERs):


Fig. 2m

The KER of 2h = 79610 show two major contributions according to the two different

ion formation mechanisms (i) and (ii) (see (1); conclusion above) (ii) broad

„underlying“ continuum feature corresponding to 2hr (ii) and (i) sharp peak on top.

(Need to figure out what transition the sharp peak corresponds to)

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160921-CH3Br(3).pptx; slides 38-40

Likely formation mechanisms corresponding to 2hr (ii) are:

a) (2hr + 1hpd) to form CH3 + Br**(Rydberg states) followed by 1hi for Br**,

which requires 4 photons in total, (fewest number of h; MOST LIKELY)

4

3

2

1

0

2.52.01.51.00.50.0

79Br+ Ry(2hv/cm-1):

79610(16.9.16)

78370(12.9.16)

77165(15.9.16)

75905(14.9.16)

72977(7.9.16)

68684(14.9.16)67273 (13.9.16)

66019 (6.9.16)

CH3+

+ Br-





13

b) (2hr,pd) to form CH3 + Br/Br* followed by 3hi of Br/Br*, which requires 5

photons in total OR

c) (2hr + 1hpd) to form CH3** + Br/Br* followed by 3hi of Br/ Br*, which

requires 6 photons in total.

Fig. 2n

Clear two components, 1) low KER and 2) high KER are observed in the 2h =

77165, 78339 and 79610 cm-1 (see Fig.2m). Most likely the low KER component is

the ion formation channel (a) (Fig. 2n) whereas the high energy KER component is the

ion formation channel (b) (Arnar will check)


Fig. 2p

CH3 + Br*

CH3 + Br

b)

a & c) CH3** + Br*CH3** + Br

CH3** + Br**

CH3 + Br+/4 hv total

(CH3** + Br+/6hv total)

CH3 + Br+/5 hv total

b)

CH3 + Br**(Ry)

2hr

ion formationmechanisms

(CH3 + Br+/4hv total)

a)c)

1.0

0.8

0.6

0.4

0.2

0.0

2.52.01.51.00.50.0

Ry(2hv/cm-1):77165(15.9.16)

CH3+

+ Br-

eV

(a)2hr+1hpd

(b)2hr,pd

Ca. Threshold for (b)(?):Ca. Thresholdfor (a)(?):


14

3) Results:

VMI-REMPI data were collected for one color excitation corresponding to two-

photon excitation to number of Rydberg states within the range of 66000 – 80000 cm-1

and electron detection, for ten wavenumber values in total:



Fig. 3a

PES spectra were derived from the images:

PES spectra:


CH3Br(4).pptx slides: 6-7

See assignments/PES spectra predictions:

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-161014-CH3Br(4).pptx (AK & AH)

Slides: 10-13 AND

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-161026-CH3Br(4).pptx (AH)

Slides: 2-4 AND

https://notendur.hi.is/~agust/rannsoknir/Crete16/WORD-161017PG.docx (Pavle)

8000078000760007400072000700006800066000

2h / [cm-1

]

[3/2]np;w

[1/2]np;w

[3/2]nd;w

[1/2]nd;w

5 6 7

0 0 02

n =

w =

5 6

02

02

0

02

02

02

02

4 6

4 5

3 2 1 3 2

3 2

3 2 1

3 2

66

01

9

68

68

4

68

46

1

72

65

5

72

97

7

75

41

87

56

86

75

90

5

78

22

5

78

37

0

78

19

3

78

40

1

79

61

0

80

64

0

80

75

8

80

67

4

80

88

1

66

50

3

67

27

5

68

88

2

69

94

7

74

24

9

76

68

97

71

65









https://notendur.hi.is/~agust/rannsoknir/Crete16/WORD-161017PG.docx

15

Conclusions:

Since CH3+

are the major ions formed the major contribution to the PES´s also are

linked to the CH3+ formation, i.e. due to the two major mechanisms:

2hr(ii) (2hr + 1hpd): CH3** + 1h -> CH3+ + e-

1h(i) (i.e. 1hpd): CH3 + 3h -> CH3+ + e-

Since the former mechanism (ii) involves one-photon ionization and the latter

mechanism (i) involves three-photon ionization it is convenient to compare the PES´s

by h (ii) and 3h shifts (i), respectively (see (2) above).

NB: The latter mechanism (i) is only significant for the 2h = 77165 and 79610 cm-1

spectra (i.e. where significant sharp peaks in the one-color KERs are observed (see (2)

above)

2hr(ii) (2hr + 1hpd) mechanism (comparison of PES´s´for 1*h shifts):

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-161014-CH3Br(4).pptx ; slide: 16:

Fig. 3b

shows,:

a) -(reasonably) good match of peaks corresponding to ionization (thresholds) of the

i) CH3**(3p2A2), ii) CH3**(3d2E) and/or CH3**(3d2A1), iii) CH3**(4p2A2) and iv)

CH3**(4f2E) for all PES´s

b) –peaks corresponding to CH3** = CH3**(3p2A2) only for the four lowest energy

excitation spectra (66019, 66503, 67275 and 68684 cm-1)

2hr(ii) (2hr + 1hpd) mechanism (comparison of PES´s´for 1*h shifts): The four

lowest energy excitation spectra (66019, 66503, 67275 and 68684 cm-1),

corresponding to resonance transitions to CH3Br**(5p) Rydberg states show

transitions for v´= -1, 0 and +1:


8

6

4

2

0

6420

5p (3/2)

5p (3/2)

5p (3/2)

4d (3/2)

5p (1/2)

4d (3/2)

6p; 5d (3/2)4d (1/2)

6p (1/2)5d (3/2)

6p (3/2)

7p (3/2)

1 hv

CH3(3p2A2)CH3(3d

2E,3d

2A1)CH3(4p

2A2)CH3(4f

2E)

h eV

CH3++Br-



16

Fig. 3c

1h (i) (1hpd) mechanism:

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-161014-CH3Br(4).pptx ; slides: 16-

18:

The highest energy structure in the PES spectrum for 2h = 77165 cm-1 (see Fig. 3b

above) matches thresholds for thee-photon excitation of CH3(v1,v2,v3,v4), i.e. for

CH3(v1,v2,v3,v4) + 3h -> CH3+( v1,v2,v3,v4) + e-

1.0

0.8

0.6

0.4

0.2

0.0

Inte

nsity /

a.u

.

2.52.01.51.00.50.0KER (eV)

CH3*(0,0,0,0)

CH3+(0,1,0,0) + e-

CH3*(0,0,0,0)

CH3+(0,0,1,0) + e-

CH3*(0,0,0,0)

CH3+(0,0,0,0) + e-

CH3*(1,0,0,0)

CH3+(0,0,0,0) + e-

From NIST values for CH3(3p2A2) and CH3

+(X):(1, 2, 3, 4)

CH3*(0,0,0,0)

CH3+(0,0,3,0) + e-

CH3*(0,0,0,0)

CH3+(0,1,3,0) + e-

Electron KER for 68684 (291.231nm) v = 0

v = -1

v = 1

CH3*(0,0,0,0)

CH3+(0,0,2,0) + e-

CH3*(0,0,0,0)

CH3+(0,1,2,0) + e-

v = 2

v = 3

(be, str, str, be)

1.0

0.8

0.6

0.4

0.2

0.0

6420 eV

Red lines indicate 3 hvtransitions between vibrational states of the CH3 fragment

Forming CH3 in ground stateThen 3 hv photon ionization

PES for 77165 cm-1


17

Possible PES´s due to CH2** + 1h -> CH2

+ + e-:

Thresholds corresponding to one-photon ionization of CH2**suggest that “the low

energy side” of a broad peak at about 1.7 eV on a 1hv shift scale shown below is due

to CH2**(3p) + 1h -> CH2+ + e-:

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-161014-CH3Br(4).pptx ; slides: 12:

0.6

0.5

0.4

0.3

0.2

0.1

0.0

5.04.84.64.44.24.0 eV

CH

3(1

,0,0

,0)

+ 3

h

-> C

H3+ (

0,0

,0,0

) +

e-

CH

3(0

,0,0

,0)

+ 3

h

-> C

H3+ (

0,0

,0,0

) +

e-(n

0)

CH

3(0

,0,0

,1)

+ 3

h

-> C

H3+ (

0,0

,0,1

) +

e-(d

ef.)

CH

3(0

,0,1

,0)

+ 3

h

-> C

H3+ (

0,0

,1,0

) +

e-(C

H s

tr.)

CH

3(0

,1,0

,0)

+ 3

h

-> C

H3+ (

0,1

,0,0

) +

e-;

OP

LA

CH

3(0

,2,0

,0)

+ 3

h

-> C

H3+ (

0,2

,0,0

) +

e-;

OP

LA

CH

3(0

,3,0

,0)

+ 3

h

-> C

H3+ (

0,3

,0,0

) +

e-;

OP

LA

CH

3(0

,4,0

,0)

+ 3

h

-> C

H3+ (

0,4

,0,0

) +

e-;

OP

LA

CH

3(0

,5,0

,0)

+ 3

h

-> C

H3+ (

0,5

,0,0

) +

e-;

OP

LA


18

Yet unassigned features in the PES´s could be due to various other ionization

channels, such as:

CH3 + 2h -> CH3+ + e- (low energy)

CH2** ->->-> CH2+ + e-

CH/CH** ->->-> CH+ + e-

Br/Br*/Br** ->->-> Br+ + e-

CBr** -> CBr+ + e-

19

4) Results:

VMI-REMPI data were collected for two color excitation corresponding to two-

photon excitation to number of Rydberg states within the range of 66000 – 80000 cm-

1 and Br and Br* detection -, for five wavenumber values in total:



Fig. 4a

KER spectra and angular distributions were derived from the images:

KER spectra:


CH3Br(3).pptx; slides 2 -42

See assignments/KER spectra predictions:


Angular distributions: https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160921-

CH3Br(3).pptx; slides 43-56

8000078000760007400072000700006800066000

2h / [cm-1

]

[3/2]np;w

[1/2]np;w

[3/2]nd;w

[1/2]nd;w

5 6 7

0 0 02

n =

w =

5 6

02

02

0

02

02

02

02

4 6

4 5

3 2 1 3 2

3 2

3 2 1

3 2

66

01

9

68

68

4

68

46

1

72

65

5

72

97

7

75

41

87

56

86

75

90

5

78

22

5

78

37

0

78

19

3

78

40

1

79

61

0

80

64

0

80

75

8

80

67

4

80

88

1

66

50

3

67

27

5

68

88

2

69

94

7

74

24

9

76

68

97

71

65










20

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160904-CH3Br(1).pptx, slides 38-39:

Br and Br* atoms were detected by (2r +1i) REMPI for the resonance transitions:

Br ->-> Br**(4P3/2); excitation = 266.6784 nm / 2h= 74996.6720 cm-1.

Br* ->-> Br**(2D3/2); excitation = 266.7420 nm / 2h= 74978.8185cm-1.

These excitations correspond to ion formation according to the 1h (i) mechanism (see

above)

Fig. 4b

Images / spectra were recorded,

a) –for both laser radiations; probe laser (Dye laser) delayed by about 10 ns with

respect to the pump laser (MOPO) beam, i.e. “two-color data”.

b) -for pump laser (MOPO) only

c) –for probe laser (Dye laser) only

Typical results were as the following (for 2h = 67275 cm-1):

see https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160921-CH3Br(3).pptx, slide 11:

Fig. 4c

7700076000750007400073000

2h / [cm-1

]

BrBr**(4P3/2)

Br*Br**(2D3/2)

[3/2]nd;w

32 [1/2]nd;w

[3/2]np;w

[1/2]np;w

Dye lasers / probe transitions

75009.1374991.41

1.0

0.8

0.6

0.4

0.2

0.0

1.00.80.60.40.20.0

KERs (xx) / Br (533.358nm) detect.:

Ry(2hv/cm-1):

67275(21.9.16)

eV

FromMOPO

FromDye laser

Both lasers



21

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160921-CH3Br(3).pptx, slide 41-43:

Thus the “two-color KERs” are typically found to be made of three major

components:

a) medium KERs with a sharp peak, b) broad high KERs and c) broad low KERs:

a) The medium KERs (a) correspond to ion formation mechanism 1h(i) (which gives

sharp peaks) primarily due to the probe / dye laser, i.e.

1hpd of CH3Br by the probe radiation to form CH3 + Br/Br* followed by the

(2r+1i)REMPI (probe) of Br/Br*

b) The broad high KERs correspond to ion formation mechanism 2hr(ii) (which gives

broad peaks) by the pump and probe radiations, i.e.

2hr,pd of CH3Br by the probe radiation to form CH3 + Br/Br* followed by the

(2r+1i)REMPI (probe) of Br/Br*

c) The broad low KERs correspond to ion formation mechanism 2hr(ii) (which gives

broad peaks) by the pump and probe radiations, i.e.

(2hr + 1hpd) of CH3Br by the probe radiation to form CH3** + Br/Br* followed by

the (2r+1i)REMPI (probe) of Br/Br*:

Fig. 4d

CH3 + Br

a)

c)CH3* + Br

c)

CH3* + Br+/6hv total

CH3 + Br+/4-5 hv total

a-b)

CH3 + Br**

CH3* + Br**

b)


22

Fig. 4e

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160921-CH3Br(3).pptx,

slide 31:

Relevent thresholds corresponding to mechanisms (b) and (c) above are shown below:

b): for CH3 + Br formation after 2hr,pd (for 2h = 66019, 67275 and 72977 cm-1)

c): for CH3**(3p2A2) + Br formation after 2hr + 1hpd (2hr = 66019, 67275 and

68684 cm-1)

c) for CH3**(3d) + Br formation after 2hr + 1hpd (2hr = 72977 cm-1)

Fig. 4f

The angular distributions for (c)/”low KERs”/Br*(Br) detection, according to the two-

color detection and the CH3**(3p2A2) formation according to one-color detection (see

above, Fig. 2j) are found to be comparable. This suggest a common formation channel

1.2

1.0

0.8

0.6

0.4

0.2

0.0

1.00.80.60.40.20.0

(b)(c)

(a)

eV

Ry(2hv/cm-1):

72977(21.9.16)

2hr,pd

1hpd2hr+1hpd

1.0

0.8

0.6

0.4

0.2

0.0

1.00.80.60.40.20.0

KERs (xx) / Br (533.358nm) detect.:

eV

Both lasers„Total signal“

Ry(2hv/cm-1):75905(22.9.16)72977(21.9.16)68684(22.9.16)67275(21.9.16)66019(21.9.16)

Normalizationpoints

Thresholds for CH3+Brformation after 2hr

Thresholds for CH3**+Brformation after 2hr + 1hpd


23

of 2hr+1hpd followed by a formation of CH3**(3p2A2) + Br*(Br):

https://notendur.hi.is/~agust/rannsoknir/Crete16/PPT-160921-CH3Br(3).pptx,

slide 57:

Fig. 4g

VS.

2

Low KER

High KER

Medium KER

Two color, Br* detectionBr detection

Summary2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

74727068

x103

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

78767472706866

x103

2

1.2932 660190.71411 665031.1593 672750.71876 686842.0226 729771.3393 74249-0.058 759050.5568 771650.2957 783700.0092239 79610

CH3+


24

5) Results:

VMI-REMPI data were collected for two color excitation corresponding to two-

photon excitation to a Rydberg state within the range of 66000 – 80000 cm-1 and CH3

detection -, for one wavenumber value (one Rydberg state) (72977 cm-1) :



KER spectra were derived from the images:

KER spectra:


Slides 2, 4-6,

8000078000760007400072000700006800066000

2h / [cm-1

]

[3/2]np;w

[1/2]np;w

[3/2]nd;w

[1/2]nd;w

5 6 7

0 0 02

n =

w =

5 6

02

02

0

02

02

02

02

4 6

4 5

3 2 1 3 2

3 2

3 2 1

3 2

66

01

9

68

68

4

68

46

1

72

65

5

72

97

7

75

41

87

56

86

75

90

5

78

22

5

78

37

0

78

19

3

78

40

1

79

61

0

80

64

0

80

75

8

80

67

4

80

88

1

66

50

3

67

27

5

68

88

2

69

94

7

74

24

9

76

68

97

71

65






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