1. Electronic spectroscopy, lifetimes and barrier to linearity in the A 1B1–X 1A1 system of CBr2
CHONG TAO, D. BRUSSE, Y. MISHCHENKO, C. MUKARAKATE and S. A. REID,
Department of Chemistry, Marquette University, Milwaukee, WI
62nd International Symposium on Molecular Spectroscopy, June 18th, 2007

2. Background on CBr2
The spectroscopy AX system of CBr2 was first detected in Ar matrix by Andrews in 1968.a
In 1990, Zhou et al. reported the first room-temperature gas phase spectrum in their cross-beam apparatus and assigned the origin at cm-1, but the bandwidth(~40cm-1) limited spectra information and precision.b
In 1993, Xu and Harmony(XH) carried out the most extensive study of CBr2, recording the jet-cooled LIF spectrum of CBr2 through pyrolysis of CHBr3, and reassigned the electronic origin to ~15092 cm-1.c
Chang and co-workers recorded the emission spectra of CBr2 in a discharge source. However, the CHBr and Br2 generated in the beam greatly interfered with the CBr2 spectrum.d
From our experience of halocarbenes, we employed an improved recipe to generate CBr2 without the interference of CHBr and Br2, by pulsed discharge of a gas mixture of CHBr3/CBr4 seeded in Ar.
a: JCP., 48, 979(1968)
b: CPL., 166, 547(1990)
c: JPC., 97, 7465(1993)
d: PCCP., 7, 2468(2005)

3. CBr2: Xu and Harmony
7466 The Journal of Physical Chemistry, Vol. 97,No.29,1993
Figure 1. Prominent progressions of A  X transition of CBr2

4. Fluorescence excitation
survey spectrum of CBr2

5. Fluorescence excitation spectrum and simulation
Individual isotopomer is simulated and C79Br2, C79Br81Br, C81Br2 are scaled to a 1:2:1 ratio
For each band, we obtained the band origins and A rotational constants for all 3 isotopomers.

6. Revision in origin assignment based on excited state isotope shifts
The isotope shifts can be
fit to the expression:
We expect that DT00 is
very close to zero
CBrCl(-0.08cm-1),
CCl2 (-0.22cm-1)
This is true if we shift
the origin position of
Xu and Harmony by one quantum, to~15,279 cm-1
DT00 =-0.02cm-1

7. Vibrational parameters of à state
T00   
Reference
Experiment
C81Br2
(48)
472.57(46)
183.11(11)
-0.539(44)
-0.021(6)
This work
(16)
473.8(12)
183.6(4)
-0.62(13)
-0.038(20) XH
C79Br81Br
(38)
472.99(35)
184.13(8)
-0.519(35)
-0.024(4)
(5)
475.0(5)
184.9(1)
-0.71(7)
-0.058(9)
C79Br2
(46)
473.58(44)
185.19(11)
-0.537(42)
-0.026(5)
(17)
474.8(12)
185.4(4)
-0.040(21)
Theory
MRCI
15237 a
CASSCF
16486
444.6
177.9
CASPT2
14191
484.0
194.6
15192 b
XH: JPC, Vol. 97,No.29, 7466, 1993
a: JCP, Vol.112, 2227, 2000
b: JPC(A), Vol.106, 1357, 2002

8. Dependence of (A-B) constant on bending quanta
A linear trend is observed
for the A rotational constants
vs. bending quantum
number
These can be
fit to the expression:
The deviation from linearity
at high energy reflects the
onset to linearity ~18800 cm-1, consistent with theoretical prediction ~ cm-1.*
* JCP, Vol.112, 2227, 2000

9. Variation of fluorescence lifetime with energy
The lifetimes show
little dependence on
Ka over the measured
range
The measured lifetimes
are consistent with
previous matrix and
gas-phase studies
Bondebey and English:*
Ar matrix measurement:14.5s
Lee**:Gas-phase at~18000 cm-1:3.4 s
*:JMS. 79, 416(1980)
**:PCCP. 5, 3859(2003)

10. SVL emission spectra
The only experimental study about the vibrational structure was carried out by Chang and co-workers by exciting (0,12,0) and (0,13,0) bands of CBr2. Surprisingly, they failed to observe the pure bending progression in the emission spectra.*
They suggested that the congested structure beginning ~3650 cm-1 about the X 1A1 origin was transitions to a 3B1 state, which is also problematic, since:
1. This is ~ 2200 cm-1 smaller than theoretical prediction of ~5900cm-1.#
2. Dunham expansion fit to the observed levels gives a small
standard deviation ~2.3 cm-1, suggesting a very small spin-orbit
interaction.
*: PCCP., 7, 2468(2005)
#: JPCA., 106, 8495(2002)

11. SVL emission spectra
Pumping K`a = 0 of various bands of C79Br81Br, we probed level up to 7000 cm-1 above X state.

12. SVL emission spectra
Polyad structure due to the near 3:1 resonance of the ground state symmetric stretching and bending frequencies.
Since only 1-2 levels in each polyad display significant FC activity, the spectrum retains a regular appearance.

13. Ground state vibrational constants
Parameter
Dunham fit 1
606.6(4) 2
199.5(2) 3
679.8(7)
11
-2.01(4)
12
-1.23(2)
13
-10.66(29)
22
-1.49(1)
23
-2.60(10)
33
Fixed at 0.0
ab initio*
601.6
196.6
655.3
Parameters of C79Br81Br
Anharmonic frequency 3
Fit 92 levels with =2.7 cm-1
*JPCA., 103, 7900(1999)

14. Summary
Using an improved recipe, LIF and SVL emission spectroscopy of A 1B1–X 1A1 system of CBr2 were studied.
The electronic transition origin was reassigned to cm-1, from isotope splitting.
Vibrational constants for both ground(X) and excited(A) states were obtained.
Fluorescence decay lifetime decrease dramatically with excitation energy and show no obvious dependence on K`a quantum number. However, a sudden increase of A rotational constant near cm-1 indicates the approach of linearity.
Small standard deviation for the fit of the X state vibrational levels up to 6000 cm-1 indicates very weak/ no spin-orbit interaction between the X and low-lying triplet state, which is predicted ~5900cm-1.*
*:JPCA., 106, 8495(2002)

15. Acknowledgements People: Calvin Mukrakate Mihaela Deselnicu
Yulia Mishchenko
Daniel Brusse
Robert Forner
Dr. Scott Reid
Dr. Timothy Schmidt
Dr. Scott Kable
Funding:
$ National Science
Foundation