Electronic spectroscopy of the A1A" - X1A' system of CDBr (original) (raw)
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Electronic spectroscopy of the à [sup 1]A[sup ʺ]↔X̃ [sup 1]A[sup ʹ] system of CDBr
The Journal of Chemical Physics, 2006
To further investigate the Renner-Teller (RT) effect and barriers to linearity and dissociation in the simplest singlet carbene, we recorded fluorescence excitation spectra of bands involving the pure bending levels 2 n 0 with n = 0-9 and the combination states 1 1 0 2 n 0 with n = 1-8 and 2 n 0 3 1 0 with n = 0-5 in the à 1 A 00 ' X 1 A 0 system of CDF, in addition to some weak hot bands. The spectra were measured under jet-cooled conditions using a pulsed discharge source, and rotationally analyzed to yield precise values for the band origins and rotational constants; fluorescence lifetimes were also measured to probe for lifetime lengthening effects due to the RT interaction. The derived à state parameters are compared with previous results for CHF and with predictions of ab initio electronic structure theory. The approach to linearity in the à state is evidenced in a sharp increase in the A rotational constant with bending excitation, and a minimum in the vibrational intervals near 2 9. A fit of the vibrational intervals for the pure bending levels yields an à state barrier to linearity in good agreement both with that previously derived for CHF and ab initio predictions. From the spectra and lifetime measurements, the onset of extensive RT perturbations is found to occur at a higher energy than in CHF, consistent with the smaller A constant.
Fluorescence excitation and emission spectroscopy of the A˜1A"-X1A' system of CHBr
We report fluorescence excitation and emission spectra of CHBr in the 450-750 nm region. A total of 30 cold bands involving the pure bending levels 2 0 n with n =2-8 and combination bands 2 0 n 3 0 1 ͑n =1-8͒, 2 0 n 3 0 2 ͑n =1-6͒, 2 0 n 3 0 3 ͑n =1-2͒, 1 0 1 2 0 n ͑n =5-7͒, 1 0 1 2 0 n 3 0 1 ͑n =4-6͒, and 1 0 1 2 0 n 3 0 2 ͑n =5͒ in the à 1 AЉ ← X 1 AЈ system were observed, in addition to a number of hot bands. The majority of these are reported and/or rotationally analyzed here for the first time. Spectra were measured under jet-cooled conditions using a pulsed discharge source, and rotational analysis yielded band origins and rotational constants for both bromine isotopomers ͑CH 79 Br, CH 81 Br͒. The derived à 1 AЉ vibrational intervals are combined with results of ͓Yu et al. J. Chem. Phys. 115, 5433 ͑2001͔͒ to derive barriers to linearity for the 2 n , 2 n 3 1 , and 2 n 3 2 progressions. The à 1 AЉ state C-H stretching frequency is determined here for the first time, and the observed 3 dependence of the 79 Br-81 Br isotope splitting in the à 1 AЉ state is in good agreement with theoretical expectations. Our dispersed fluorescence spectra probe the vibrational structure of the X 1 AЈ state up to ϳ9000 cm −1 above the vibrationless level; the total number of levels observed is more than twice that previously reported. As first reported by ͓Chen et al. J. Mol. Spectrosc. 209, 254 ͑2001͔͒, these spectra reveal numerous perturbations due to spin-orbit interaction with the low-lying ã 3 AЉ state. The results of a Dunham expansion fit of the ground state vibrational term energies, and comparisons with previous experimental and theoretical studies, are reported. Our results lead to several revised assignments, including the X 1 AЈ C-H stretching fundamental. Globally, the vibrational frequencies of X 1 AЈ, ã 3 AЉ, and à 1 AЉ are in excellent agreement with theoretical predictions.
Visible Wavelength Electronic Spectroscopy of the DCBr ÖX̃ Vibronic Transition
Journal of Molecular Spectroscopy, 2001
The laser excitation and dispersed fluorescence spectra of the DCBrÃ-X vibronic bands between 570 and 600 nm were successfully obtained in a direct current (DC) discharge supersonic free jet expansion. The analysis of the dispersed fluorescence spectra results in information on the vibrational structure of the DCBrX 1 A state for the first time. Perturbations due to the background levels of theã 3 A state can be recognized. The data are consistent with the previously reported triplet-singlet energy gap (the zero-point level energy gap) of 5.73 ± 0.02 kcal/mol for HCBr. We have determined the vibrational frequencies to be 832 cm −1 (bend) and 663 cm −1 (C-Br stretch) for theX 1 A state. In addition to theX state vibrational structure, theÃ(0,6,2) andÃ(0,5,0) levels were also observed for the first time.
Using an improved discharge recipe for the production of dibromocarbene (CBr 2 ), we recently reassigned the electronic origin of thẽ A 1 B 1 X 1 A 1 system, based on an extensive data set of isotope shifts for the pure bending transitions [C. Tao, C. Mukarakate, D. Brusse, Y. Mishchenko, S. A. Reid, J. Mol. Spectrosc. 240 (2006) 139-140]. In this study, we report the complete analysis of the fluorescence excitation spectrum of theà 1 B 1 X 1 A 1 system in the 525-650 nm region, obtained at a rotational temperature of 10K.Atotalof32coldbandsinvolvingthepurebendinglevels2n0withn=2−19andcombinationbands1102n0(n=1−16)intheA1B1X1A1systemwereobserved;anumberofthesearereportedhereforthefirsttime.RotationalanalysistypicallyyieldedArotationalconstantsandbandoriginsforallthreebromineisotopomers(C79Br2,C79Br81Br,C81Br2),andDunhamexpansionfitsyieldedanextensivesetofvibrationalparametersforeach.Theisotopeshiftsareingoodagreementwiththeproductrule,and,whenplottedvs.bendingquantumnumber,themeasuredAconstantsfollowalineartrendexceptforthehighestbendinglevels,whereanabruptincreaseisobserved,indicativeoftheapproachtolinearity.Thisismirroredinthevibrationalintervals,whichalsochangeabruptlyinthisregion,andtheestimatedbarrierheightof10 K. A total of 32 cold bands involving the pure bending levels 2 n 0 with n = 2-19 and combination bands 1 1 0 2 n 0 (n = 1-16) in theà 1 B 1 X 1 A 1 system were observed; a number of these are reported here for the first time. Rotational analysis typically yielded A rotational constants and band origins for all three bromine isotopomers (C 79 Br 2 , C 79 Br 81 Br, C 81 Br 2 ), and Dunham expansion fits yielded an extensive set of vibrational parameters for each. The isotope shifts are in good agreement with the product rule, and, when plotted vs. bending quantum number, the measured A constants follow a linear trend except for the highest bending levels, where an abrupt increase is observed, indicative of the approach to linearity. This is mirrored in the vibrational intervals, which also change abruptly in this region, and the estimated barrier height of 10K.Atotalof32coldbandsinvolvingthepurebendinglevels2n0withn=2−19andcombinationbands1102n0(n=1−16)intheA1B1X1A1systemwereobserved;anumberofthesearereportedhereforthefirsttime.RotationalanalysistypicallyyieldedArotationalconstantsandbandoriginsforallthreebromineisotopomers(C79Br2,C79Br81Br,C81Br2),andDunhamexpansionfitsyieldedanextensivesetofvibrationalparametersforeach.Theisotopeshiftsareingoodagreementwiththeproductrule,and,whenplottedvs.bendingquantumnumber,themeasuredAconstantsfollowalineartrendexceptforthehighestbendinglevels,whereanabruptincreaseisobserved,indicativeoftheapproachtolinearity.Thisismirroredinthevibrationalintervals,whichalsochangeabruptlyinthisregion,andtheestimatedbarrierheightof18 800 cm À1 above the vibrationless level of theX 1 A 1 state is in excellent agreement with the ab initio prediction of Sendt and Bacskay [K. Sendt, G.B. Bacskay, J. Chem. Phys. 112 (2000) 2227-2238. We also report fluorescence lifetimes as a function of vibrational level and K 0 a ; the lifetimes decrease rapidly with energy, but display no dependence on K 0 a over the measured range. The implications of these results for understanding the excited state structure of this prototypical carbene are emphasized.
Electronic spectroscopy, lifetimes, and barrier to linearity in the system of dibromocarbene
Journal of Molecular Spectroscopy, 2007
Using an improved discharge recipe for the production of dibromocarbene (CBr 2), we recently reassigned the electronic origin of thẽ A 1 B 1 X 1 A 1 system, based on an extensive data set of isotope shifts for the pure bending transitions [C. Tao, C. Mukarakate, D. Brusse, Y. Mishchenko, S. A. Reid, J. Mol. Spectrosc. 240 (2006) 139-140]. In this study, we report the complete analysis of the fluorescence excitation spectrum of theà 1 B 1 X 1 A 1 system in the 525-650 nm region, obtained at a rotational temperature of 10K.Atotalof32coldbandsinvolvingthepurebendinglevels2n0withn=2−19andcombinationbands1102n0(n=1−16)intheA1B1X1A1systemwereobserved;anumberofthesearereportedhereforthefirsttime.RotationalanalysistypicallyyieldedArotationalconstantsandbandoriginsforallthreebromineisotopomers(C79Br2,C79Br81Br,C81Br2),andDunhamexpansionfitsyieldedanextensivesetofvibrationalparametersforeach.Theisotopeshiftsareingoodagreementwiththeproductrule,and,whenplottedvs.bendingquantumnumber,themeasuredAconstantsfollowalineartrendexceptforthehighestbendinglevels,whereanabruptincreaseisobserved,indicativeoftheapproachtolinearity.Thisismirroredinthevibrationalintervals,whichalsochangeabruptlyinthisregion,andtheestimatedbarrierheightof10 K. A total of 32 cold bands involving the pure bending levels 2 n 0 with n = 2-19 and combination bands 1 1 0 2 n 0 (n = 1-16) in theà 1 B 1 X 1 A 1 system were observed; a number of these are reported here for the first time. Rotational analysis typically yielded A rotational constants and band origins for all three bromine isotopomers (C 79 Br 2 , C 79 Br 81 Br, C 81 Br 2), and Dunham expansion fits yielded an extensive set of vibrational parameters for each. The isotope shifts are in good agreement with the product rule, and, when plotted vs. bending quantum number, the measured A constants follow a linear trend except for the highest bending levels, where an abrupt increase is observed, indicative of the approach to linearity. This is mirrored in the vibrational intervals, which also change abruptly in this region, and the estimated barrier height of 10K.Atotalof32coldbandsinvolvingthepurebendinglevels2n0withn=2−19andcombinationbands1102n0(n=1−16)intheA1B1X1A1systemwereobserved;anumberofthesearereportedhereforthefirsttime.RotationalanalysistypicallyyieldedArotationalconstantsandbandoriginsforallthreebromineisotopomers(C79Br2,C79Br81Br,C81Br2),andDunhamexpansionfitsyieldedanextensivesetofvibrationalparametersforeach.Theisotopeshiftsareingoodagreementwiththeproductrule,and,whenplottedvs.bendingquantumnumber,themeasuredAconstantsfollowalineartrendexceptforthehighestbendinglevels,whereanabruptincreaseisobserved,indicativeoftheapproachtolinearity.Thisismirroredinthevibrationalintervals,whichalsochangeabruptlyinthisregion,andtheestimatedbarrierheightof18 800 cm À1 above the vibrationless level of theX 1 A 1 state is in excellent agreement with the ab initio prediction of Sendt and Bacskay [K. Sendt, G.B. Bacskay, J. Chem. Phys. 112 (2000) 2227-2238]. We also report fluorescence lifetimes as a function of vibrational level and K 0 a ; the lifetimes decrease rapidly with energy, but display no dependence on K 0 a over the measured range. The implications of these results for understanding the excited state structure of this prototypical carbene are emphasized.
The Journal of Chemical Physics, 1999
Pump-probe spectra of HeBr 2 in vibrational states vЈϭ10 and 39 through 48 of the B electronic state are reported and the fragment rotational distributions from vibrational predissociation of the cluster are extracted from the measured E(0 g ϩ)←B(3 ⌸ 0u ϩ) spectra of Br 2. The experimental results are compared to theoretical calculations on the B←X spectra using atom-atom model potentials and performing a thermal average over transitions that contribute to the net excitation. Very good agreement between experiment and theory is obtained, except in the region of vЈϭ44, where the ⌬vϭϪ1 channel closes, and in the region of vЈϭ48 where the ⌬vϭϪ2 channel closes. For vЈ ϭ43, and vЈϭ44, the agreement is less satisfactory because the dynamics are extremely sensitive to details of the potential energy surface due to threshold effects associated with the ⌬vϭϪ1 channel closing. Similar sensitivity to the potential due to the ⌬vϭϪ2 channel closing impairs the agreement between experiment and theory for vЈϭ48. Below vЈϭ43, the rotational distributions for ⌬vϭϪ1 and ⌬vϭϪ2 are quite similar. Above vЈϭ43 the peaks of the rotational distributions for ⌬vϭϪ2 move to higher values of j. These results are compatible with the theoretical conclusion that dissociation shifts from a direct mechanism to one involving intramolecular vibrational distribution in the region of the closing of the ⌬vϭϪ1 channel. Although the simple additive potential model used in this work succeeds in reproducing most of the experimental data for this system, further improvements in the potential energy surface will be required to achieve precise agreement between experiment and theory for large Br-Br separations.
Journal of Molecular Spectroscopy, 2017
The rovibrational spectrum of Deuterobromochlorofluoromethane CDBrClF has been measured with a resolution of better than 0.001 cm À1 at room temperature with the Zurich Bruker IFS spectrometer prototype (ZP 2001) using a globar source, and with the Bruker prototype 2009 interferometer connected to the Swiss Light Source (SLS) synchrotron as a radiation source providing a resolution of 0.0007 cm À1 in the range 600-1100 cm À1. In addition CDBrClF has been measured at low rotational temperature (T rot = 15 K) with our diode laser supersonic jet setup around 748 cm À1 in the m 5 region (resolution 0.0013 cm À1). The spectra of the two major isotopomers CD 79 Br 35 ClF and CD 81 Br 35 ClF have been analyzed within the m 5 (CCl-stretch,m 0 ¼ 748:2998 cm À1 (CD 79 Br 35 ClF),m 0 ¼ 748:1541 cm À1 (CD 81 Br 35 ClF)), m 4 (CFstretch,m 0 ¼ 1082:8116 cm À1 (CD 79 Br 35 ClF),m 0 ¼ 1082:7963 cm À1 (CD 81 Br 35 ClF)) and m 3 (DCF-bend, m 0 ¼ 918:3715 cm À1 (CD 79 Br 35 ClF),m 0 ¼ 918:0206 cm À1 (CD 81 Br 35 ClF)) regions. A detailed rovibrational analysis of these bands is presented. A combined analysis of the m 3 ; m 4 and m 5 fundamental bands provides accurate rotational and quartic centrifugal distortion constants for the vibrational ground state and for the states m 3 ; m 4 and m 5 of CD 79 Br 35 ClF and CD 81 Br 35 ClF. The results are discussed in relation to intramolecular (ro)vibrational redistribution (IVR) and also possible experiments pertaining to the experimental detection of molecular parity violation.
The Journal of Chemical Physics, 2000
The rovibrational spectra of deuterobromochlorofluoromethane ͑CDBrClF͒ were measured at intermediate ͑0.1 cm Ϫ1 ͒ and high resolution ͑0.0024 cm Ϫ1 full bandwidth, half-maximum͒ by interferometric Fourier transform infrared spectroscopy in the range from the far infrared at 200 cm Ϫ1 to the near infrared ͑12 000 cm Ϫ1 ͒ covering all the fundamentals and CD stretching overtones up to polyad Nϭ5. The spectra are completely analyzed in terms of their vibrational assignments to fundamentals, combinations and overtones. At high excitation the analysis reveals the dominant anharmonic coupling between four high frequency vibrational modes; the CD stretching (1), two CD bending (2 , 3), and the CF stretching mode (4). The analysis is carried out using effective model Hamiltonians including three and four vibrational degrees of freedom. We also present vibrational variational calculations on a grid in a four-dimensional normal coordinate subspace. The potential energy and the dipole moment function are calculated ab initio on this grid using self-consistent field second order Møller-Plesset perturbation theory ͑MP2͒. Experimental and theoretical results for band positions and integrated intensities as well as effective spectroscopic parameters are found to be in good agreement. The important anharmonic coupling between the CD chromophore and the CF stretching vibration can be described by an effective cubic Fermi resonance coupling constant k s f f Ј Ϸ(50Ϯ10) cm Ϫ1 , which leads to intramolecular vibrational redistribution between the CD and CF chromophores on the femtosecond time scale. Time dependent intramolecular vibrational redistribution processes in CDBrClF are derived in various representations, including time dependent probability densities ͑''wave packets''͒ in coordinate space and finally time dependent entropy.
The Journal of Chemical Physics, 2002
The B←X rovibronic excitation spectrum of the HeBr 2 van der Waals complex is calculated using an ab initio potential energy surface for the ground electronic state. The coupled-cluster single double triple calculations predict double-minimum topology ͑linear and T-shaped wells͒ for the X-state potential with a low isomerization barrier. The two lowest vibrational levels, assigned to T-shaped and linear isomers using the localization patterns of the corresponding wave functions, are almost degenerated and lie slightly above the isomerization barrier. This indicates that T-shaped and linear isomers can coexist even at low temperatures and give rise to two separated bands in the excitation spectrum. The main band of the B←X excitation spectrum is assigned to transitions from the T-shaped isomer, whereas the very good agreement between the observed and calculated spectrum, using the ab initio X-state potential, demonstrates that the unassigned secondary band corresponds to excitation of the linear isomer of the HeBr 2 (X) complex. The complete assignment of the spectrum in terms of individual rovibronic transitions is presented.