the a(0+)−x(0+) transition of bibr: radiative lifetimes of the a state

ELSEVIER Chemical Physics 199 (1995) 305-311

Chemical Physics

The A( 0 ÷) -X( 0 ÷) transition of BiBr: radiative lifetimes of the A state

E. Mart[nez *, M.R. L6pez, A. Notario, F. Poblete Departamento de Qufmica Ffsica, Facultad de Qufmicas, Universidad de Castilla-La Mancha, Campus Universitario s / n,

13071-Ciudad Real, Spain

Received 13 February 1995

Abstract

Laser induced fluorescence from resolved rovibrational levels (v', J ' ) of the electronically excited A(0 +) state of BiBr has been observed. Fluorescence lifetimes for several rovibrational levels (v', J ' ) of the BiBr A(0 +) state in the v' = 4-9 interval have been measured, and a study of their variation with pressure has been made. No significant variations of radiative lifetimes from specific vibrational levels v' (v' = 0-9) of the BiBr A excited state has been observed, and no dependence on the rotational energy, within a vibrational level, was detected for the measured lifetimes. A collision-free radiative lifetime of 70 = 0.52 I~s is proposed for the BiBr A(0 +) state and values for quenching rate constants in the order of ko-----(1.0 Jr 0.3)× 10 - l l cm 3 molecule-~ s-~ in the spectral range studied were obtained. From calculated Franck- Condon factors for the v ' - v" transitions of the A-X system and from our radiative lifetimes, electronic transition moments have been calculated.

1. Introduct ion

To our knowledge the literature on BiBr is rather scarce. A BiBr electronic spectrum was first observed in emission by Howell and Rochester [1] and later in absorption by Morgan [2]. Sankaranarayanan et al. [3] and Singh [4] performed the vibrational analysis o f several bands o f the A - X system. Lal et al. [5] made the rotational analysis of seven bands in the A - X system. Kuijpers and Dynamus [6] calculated some rotational constants for the ground state from microwave absorption spectra of Bi79Br and BiSlBr. Pati~o et al. [7] observed 13 fluorescence series in the spectra excited by the 4765 and 4880/k lines of an a rgon- ion laser in Bi79Br and Bi 81Br.

* Corresponding author.

No attention had been paid to the study of the dynamics of the electronically excited states of the bismuth halides BiCI, BiBr and BiI. However, since the interest raised by the A - X transition of BiF as a chemical laser, studies of the radiative lifetime and kinetics of the A 0 ÷ state of BiF were carried out [8-11], and similar investigations of the other BiX molecules were required. With this aim a study of the radiative lifetimes and dynamics of the excited states of BiCI have been previously carried out in our laboratory [12].

As far as we know, no determinations of fluorescence lifetimes o f the A electronic excited state o f BiBr have been reported, neither have collision cross section efficiencies for deactivation of this excited state been evaluated, except for our first preliminary results [13]. No values have been found in the litera-

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306 E. Martlnez et al. / Chemical Physics 199 (1995) 305-311

ture for the electronic transition moment of the A - X system of BiBr. Laser induced fluorescence (LIF) provides a natural way of studying the dynamics of quantum-selected states of electronically excited molecules. Pulsed excitation enables one to measure the fluorescence decay in real time leading to rate constants for both collision-free and collisional pro- cesses. In a previous work [13] fluorescence lifetimes for low-lying vibrational levels (v = 0-3) of the A (0 ÷) state of BiBr were given for the first time by laser induced fluorescence. However, no study of the rotational energy was made, and the vibrational interval studied was very short and no information was obtained on higher vibrational levels of the A state in order to study the dynamics of this excited state of the BiBr molecule.

The nature and excitation energy of the low-lying excited states of the bismuth halides, is a matter of controversy. For the most studied bismuth halide, the BiF molecule, the experimental interpretations of Ra t and Rat [14] and Jones and Mclean [15] have identified the A(0 ÷) state with a tr2'rr3"tr .3 electronic configuration, i.e. a "rr ~ "rr * excitation relative to the ground state. However, Balasubramanian [16] considers that this state originates from the same configuration as the ground state (tr2"trarr .2) and Alekseyev et al. [17] consider that neither of these configurations is dominant for the A(0 ÷) state near its equilibrium bond distance, but rather the O'2T~4TI'*O "* configuration, corresponding to a 7 "

ti * excitation relative to the electronic ground state. The knowledge of the radiative lifetimes of this state, can be helpful in order to clear the nature of the A(0 +) excited state. Not much is known about the nature of the A(0 +) state of BiBr responsible for the A(0 +) ~ X transition, however, a similar behavior as the analogous A(0 ÷) of BiF is expected.

As part of an extensive study of the spectroscopy and dynamics of the excited states of the bismuth halides (BiCI, BiBr, BiI) carried out by this laboratory [12,13], time-resolved laser induced fluorescence has been used in this work to measure lifetimes for emission from rovibrational levels of the BiBr molecule in the v' = 4 - 9 interval of vibrational levels. Special attention has been paid to obtaining the zero-pressure lifetimes and quenching rate constants for deactivation from individual rovibrational levels of the electronic A state of this molecule.

From calculated Franck-Condon factors for the v'-v" transitions of the A - X system and from our radiative lifetimes, electronic transition moments have been calculated.

2. Experimental

The experimental setup has been described in previous publications [12,13,18,19]. The fluorescence from rovibrational levels of the A(0 ÷) state of BiBr molecule was excited by a Quantel TDL50 dye laser (pumped by a Nd-YAG 581C Quantel laser) giving pulses of 6 ns duration with a narrow linewidth of 0.08 c m - z and 2 -6 mJ of energy in the 4630-4820

region using methanol solutions of several dyes (Coumarin 460 and Coumarin 480).

Bismuth monobromide molecules were prepared by heating BiBr 3 (GFS Chemicals, 99%) and metallic bismuth (Aldrich Chem. Co., 99.999%) inside an evacuated stainless steel cell in a double oven at different temperatures (400-500°C) to control the BiBr pressure according to the following equilibrium [20]:

½BiBr 3 (g) + ]Bi (1) ~ BiBr (g) ,

where the equilibrium constant is K = p ( B i B r ) / a(BiBr)2/3p(BiBr3) 1/3. The activity a(Bi) of liquid bismuth is assumed to be unity. The BiBr vapor pressure was calculated from the equilibrium constants and the BiBr 3 vapor pressures at different temperatures were obtained by Cubicciotti and Keneshea [20,21]. BiBr 3 and metallic bismuth were deoxygenated before use. Pressure was controlled by a capacitance manometer (Leybold Inficon CM100- G10A) operating in the 10-3-10 Torr range.

When the laser was used in a wavelength scan- ning mode and undispersed fluorescence was col- lected, an excitation spectrum of the A - X band system was obtained. Alternatively fixed-frequency laser excitation was used, and the fluorescence decay was recorded in real time. Laser induced fluorescence (LIF) was detected perpendicular to the laser beam axis by a high-gain fast photomultiplier (EMI 9816B). Cut-off filters were placed between the fluorescence cell and the photomultiplier in order to block the scattered laser light. Laser excitation spectra were obtained by recording fluorescence as a

E. Mart{nez et aL / Chemical Physics 199 (1995) 305-311 307

function of the laser wavelength, using a boxcar integrator (Stanford Research Systems SR250) and a strip chart recorder. A Tektronix 2432A digital oscil- loscope was used to acquire the fluorescence decays from the photomultiplier output. Fluorescence lifetimes were obtained after averaging and processing the signals with a microcomputer connected to the system by means of a S3FG100 GURU II PLUS interface.

3. Results and discussion

Table 1 Set of levels studied, spectral range studied, rotational interval assigned and overlapping observed for Bi79Br A-X system

v ' - v" Studied spectral Assigned rotational Observed Band interval (nm) interval (J) overlapping

4-1 480.6-481.4 50-80 P(J)+ R(J + 10) 5-0 473.0-475.0 40-70 P(J)+ R(J + 10) 5 -2 481.3 -482.4 - 6-0 470.5-473.2 50-80 P(J) + R(J + 8) 7-0 467.9-471.0 50-100 P(J)+R(J +7) 8-0 465.6-467.4 30-80 P(J) + R(J + 6) 9-0 463.4-465.6 -

Laser excitation spectra of the BiBr A - X system were obtained by collecting total emission following laser excitation in the 4630-4820 ,~ region, where several lines of the 4-1, 5-0, 5-2, 6-0, 7-0, 8-0 and 9 - 0 bands of the BiBr A - X system were observed. Our spectroscopic analysis has been made only on Bi79Br. Assignments were made by calculat- ing the transition frequencies based on the constants of Patifio et al. [7,22] and Singh [4]. The spectra were calibrated by fitting the bandheads to the litera-

ture values. Fig. 1 shows a short section of the o

excitation spectrum between 4682 and 4694 A, where several rotational lines of the 7 -0 band of the Bi79Br isotopic species have been assigned. Unfortunately, the precise rotational assignment was not possible in all of these bands.

BiBr molecules were excited to rovibrational levels of the A(0 ÷) state by tuning the laser wavelength to a suitable absorption line of the A - X transition. Table 1 shows the whole set of levels studied, the

P I~e~ch SO 00 ?0 I o : ; : : : : : : '. : : ; : : : : : : '. ; :

l i t ~ a r

8O

! i ! 4m12 ~ ~ 4&g, t

Fig. 1. Laser excitation spectrum of the 7-0 (v'-v") band of BiBr A-X system, showing the assignment of the Bi 79 Br isotopic species.

308 E. Mart(nez et al. / Chemical Physics 199 (1995) 305-311

spectral range observed, rotational interval assigned and the accidental overlapping observed with a pre- cision of J _ 2. As can be seen, most of the assigned bands showed accidental overlapping of the P and R lines, so no emission from a single rotational level for these v' vibrational levels could be observed. This work completes the information given previously by other authors [3-5,7,23] over more than 100 v ' -v" vibrational bands with v' ~< 19 and v" <~

35, and also work done in this laboratory over the v ' = 0 - 3 vibrational levels of the BiBr A excited

state [ 13]. Alternatively, rotational assignments in the v ' - v"

vibrational bands were made by the method of combination differences and from the calculated vibrational isotopic shifts. Sets of lines picked out satis- fied the rotational energy combination differences relations [24]

R ( J ) - P ( J ) = (4Bv, - 6 D , , , ) ( J + 1 / 2 ) , (1)

R ( J - 1) - P ( J + 1) = ( 4 B v , - 6 D o , ) ( J + 1 / 2 ) .

(2)

Rotational constants were calculated from these relations (1,2) (see Table 2) within the J assignment

t uncertainty already mentioned. Our B~, B 5 and B~ values are slightly lower than values reported by Patifio et al. [7]. A B' 7 value is presented for the first

time. Fluorescence decay curves obtained from the time

behavior of the BiBr A - X (v ' , v") emission showed a monoexponential behavior characterized by a single lifetime around ~-= 0.5 ~s for low BiBr pressures (1 mTorr). Data could be fit to times longer than three lifetimes in all cases. The lifetime errors were evaluated as twice the standard deviation ( + 2 or) of the measurements. As an example, Table 3 shows measured lifetime values for different rotational levels of the v' = 7 vibrational level of the A

Table 2 Rotational constants calculated in this work for some vibrational levels of the A(0 + ) and X(0 + ) electronic states of BiBr molecule

v' - v" B o, (cm - 1 ) B~" (cm - t )

4-1 0.0347 4- 0.0008 - 5-0 0.0343 4- 0.0006 - 6-0 0.0341 4- 0.0009 0.0389 4- 0.0005 7-0 0.0338 4- 0.0007 0.0380 4- 0.0009

Table 3 Radiative fluorescence lifetimes (~-) values for different rovibrational levels of Bi79Br A state (Paicl = 1 mTorr). Second column shows the rotational number J' from which radiative fluorescence from A excited state is emitted, as mentioned in the text. Overlap- ping of R(J) and P(J) lines leads to simultaneous emission of two different rotational levels, as indicated in this table

v' J ' rob s (l~s)

7 52, 61 0.51 +0.2 60, 69 0.49 _+ 0.2 66, 75 0.49 _+ 0.2 74, 83 0.51 +0.1 85, 94 0.53 +0.2 94, 103 0.52_+0.1

state. Within the experimental error, no significant variation of the lifetimes with rotational energy was observed for different vibrational levels ( v ' = 4, 5, 6, 7, 8) of the BiBr A excited state studied in this work.

Collision-free lifetimes and quenching rate constants were determined by measuring fluorescence lifetimes in the range of total pressures from 0.8 to 2.6 Torr and by extrapolation to zero pressure. Fig. 2 shows a typical S te rn-Volmer plot for emission from the v ' = 5 vibrational level of BiBr A(0 ÷) state, where inverse lifetimes are plotted against total pressure, giving the total deactivation rate constants kQ and the radiative lifetime % through the relation

1 1 - = - - + k Q P T . ( 3 ) T T O

As can be seen in Fig. 2, the S te rn -Volmer plot was linear in the studied pressure interval.

2 . 3 "

't~ 2 2 . .,,t

~'~ 2.1

z

1 .9

~ . 8 . , o

P / T o r r T

Fig. 2. Stern-Volmer plot of the inverse lifetime (r) against total pressure (PT), for emission from v' = 5 vibrational level of BiBr A(0 + ) state.

E. Mart[nez et al. / Chemical Physics 199 (1995) 305-311 309

Table 4 Collision-free lifetimes (r 0) and quenching rate constants (kQ) for the set of studied levels of the A state of Bi79Br. Second column specifies the studied band of the A-X system v' v '-v" ~'0 (p,s) kQ ×10 tt (cm 3 molecule - t s - t )

4 4-1 0.56±0.02 0.9±0.1 5 5-0 0.53±0.02 0.9±0.1 6 6-0 0.52±0.03 1.0±0.3 7 7-0 0.52±0.01 1.2±0.1 8 8-0 0.49±0.02 1.1±0.3 9 9-0 0.49±0.02 a _

a Lifetime obtained at 0.8 Torr, no value at lower pressure has been obtained.

Table 4 shows the collision-free lifetimes and the quenching rate constants obtained for several vibrational levels o f the BiBr electronic A state. Laser induced fluorescence from the studied vibrational levels of the A(0 +) state gives collision-free lifetimes with a mean value of ~'0 = 0.52 Izs, as can be seen in Table 4, and this value is the proposed radiative lifetime of the A(0 ÷) state of BiBr. No significant variation in the lifetimes is observed in the vibrational interval from v' = 0 to v' = 9, neither with rotational quantum number J ' , within each vibrational level, excluding any perturbative effect by other states in this energy region. Values for quenching constants rates, kQ, of (1.0 + 0.3) × 10 - l l cm 3 molecule -~ s -1 in the spectral range studied were obtained, which shows that the efficiency of the deactivation process is not really high. This result is similar to that observed for deactivation o f the electronic states A 3• - (0+) and A~ IX+(0+) of BiC1, where values between (0.3-2.6) × 10- I I cm 3 molecule -1 s - l and around 1.3 × 10 - l l cm 3 molecule - l s - l , respectively, were found in a previous study [12].

In the literature FCF exist for several v ' - v" vibrational transitions of the BiBr A - X electronic transition, with v' = 0 - 9 and v" = 0 - 2 5 [22]. RKR curves were calculated for the X and A states, based on the vibrational constants previously reported [4,7,22]. The ground state curve was calculated with the vibrational constants of Patifio et al. [7] and Huber and Herzberg [25]. In this work, FCF and r-centroids have been calculated for the A - X system for v ' -v" vibrational transitions with v' ~< 11 and v" < 39 [26]. The spectroscopic constants for the A state potential were those given by Singh [4], obtained from the

analysis of vibrational levels 0 < v ' < 6 of the A state. Even though the potential energy curve and FCF from these data are strictly valid ones for v ' < 7, not much error should be introduced when extending the calculations up to d = 9, the value to which lifetimes have been measured and FCF calculations have been made.

The electronic transition moment, I Rel 2, is a more fundamental measure of the transition probabil- ity than the reciprocal lifetime 1/%,. The magnitude of IRe 12 for the A - X transition of BiBr is related to the measured %, value by

1 64~r 4 2 , , 3

--= ~ EIRel q,, v;,:, (4) %, 3h 0,

where q,,,, is the FCF for the v ' - v" transition. To 2 • • evaluate I Rel from %, ~t ~s necessary to know the

form of the variation of I Rel 2 with v' and v", or more precisely with r-centroid. The dependence of I Rel 2 upon r-centroids is not known for the A - X electronic transition of BiBr. Thus we considered I Rel 2 aproximately independent o f the r-centroids and the above expression can be used in the form:

1 64"tr 4 = iRe12 2 . 3 - - ~ q o ' . v ; , , , . . (5)

"1",, 3h v"

and we can calculate I Rel 2 from %, by evaluating the simple summation contained in Eq. (5) (see Table 5).

A mean value of I R e 1 2 = 0 . 9 + 0 . 1 D 2 is obtained for the calculated IRe] 2 from v' = 0 to v' = 9,

Table 5 Fluorescence radiative lifetimes (To), values of E : q,,'d' v)~,, (see text) and electronic transition moments [Re 12 for different vibrational levels (v') of BiBr A-X electronic transition v' ~'o (I s's) 10-12 ~o" qd: v3 (cm-3) IRel 2 (D2)

0 0.55+0.02 a 6.8249 0.84+0.03

1 0.53+0.01 a 6.7218 0.89+0.02

2 0.53+0.01 a 6.6194 0.91 +0.02

3 0.535:0.02 a 6.5182 0.92+0.03

4 0.564-0.02 b 6.4170 0.894-0.02

5 0.53+0.02 b 6.3174 0.95+0.03

6 0.52+0.03 b 6.2184 0.99+0.06

7 0.52+0.01 b 6.1199 1.00+0.02

8 0.49 + 0.02 b 6.0222 1.08 __. 0.04

9 0.49+0.02 b 5.9242 1.10+0.04

a Previous work [12] b This work.

310 E. Martlnez et al. / Chemical Physics 199 (1995) 305-311

1.3

0.8 I 0 .7 I I I I I I I i I I

0 1 2 3 4 5 6 7 8 9 10

V'

Fig. 3. Plot of variation of the electronic transition moment (IRe[ 2) with the v' vibrational level for the A(0 + ) -X transition of BiBr.

with a slightly higher value of I Rel 2 as v' increases (see Fig. 3). This is in good agreement with the value observed for BiF, where the magnitude of the A(0+)-XI (0 ÷) moment increases almost linearly with r e [17]. The reported value for IRe l 2 should be taken as a first aproximation to indicate the strength of the A(0+)-X(0 +) electronic transition. Obvi- ously, for a more precise evaluation, the r-centroid dependence ought to be known, and a better defini- tion of the potential energy curves is required.

When comparing the radiative lifetime obtained for the A(0 +) electronic state of BiBr, namely 0.52 p~s, with the values previously reported for the analogous low-lying electronic states of the BiCI molecule, i.e. r = 0.64 I~s for the A(0 ÷) state and z = 2.75 txs for the ,~(0 +) state, similar lifetimes are observed, especially for emission from the low-lying triplet excited states of both molecules. Reported radiative lifetimes for the low-lying levels of BiF A(0+), of around 1.4 Ixs [11], are in the same range of values, as well as unpublished preliminary lifetimes of 0.6 txs for the A(0 ÷) state of BiI. So, from these results, it can be concluded that there is little dependence on the molecular mass of the halogen atom in BiX (X = F, C1, Br, I) for the radiative lifetimes of the lowest A electronic excited states.

Recently, Alekseyev et al. found that the lowest 0 ÷ excited state, A(0+), of BiF contains a large contribution from the ,r * --* ~ * 3~r A-s state, especially at bond distances which are equal or greater than the equilibrium value for the XI(0 +) ground state. Experimentally measured lifetimes of about 2 i~s [27] for emission from the A(0 +) state of BiF, in good agreement with the calculated valued of 0.97

~s by Alekseyev, suggest a relatively large value of the electric dipole transition moment for the involved transition, as corresponding to a parallel transition. Normally the A(0 ÷) state, i.e. the lowest-lying excited 0 ÷ species, is composed almost exclusively of the b ~ A-s state. Transitions between b ~2~ ÷ and the spin-perturbed 3X- ground state are probably weaker than those involving a 37r0+ upper state. Anomalous behavior of the BiF molecule thus turns out to be explained in terms of the unusually high admixture of the 3 A-s state in the lowest-lying 0 ÷ excited state of this molecule. In this context the relatively low radiative lifetimes found for emission from the A(0 ÷) state of BiBr, in the order of the lifetimes found for BiF, suggest equally a large amount of 3rr character in the upper state of the A(0+)-X transition of BiBr.

Concerning the strength of the transition, the value of IRe[ 2= 0.9 D E obtained for the A(0+)-X(0 +) transition of BiBr, is clearly higher than those obtained for the A 3 X - ( 0 + ) - X 3 X - ( 0 + ) and A~ IE+(0+)-X 3S.-(0+) transitions of BiC1, of 0.56 and 0.148 D 2, respectively. When comparing with similar transitions in the homonuclear parent molecules Bi 2 and Br~ the value obtained is half-way between the 1.96 D E [28] and 0.2 D E [29] values for

3 + 1 + the A(0ff) 'X(0g) and B II(0 u ) - X 2~g transitions of Bi 2 and Br 2, respectively. In summary, for the first time a radiative lifetime of the A(0 ÷) state of BiBr, namely r = 0.52 I~s, is given and this value does not vary within experimental error for the v' levels (v' = 0 - 9 ) investigated and does not show variation with the rotational energy. A first value for the electronic transition moment of IRe[ 2 = 0.9 D 2 of the A -X system is derived.

Acknowledgement

MRL wishes to thank the Spanish Ministry of Education and Science for a grant during the early stages of the project.

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the a(0+)−x(0+) transition of bibr: radiative lifetimes of the a state

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