New electronic spectra of the HCCl and DCCl A-X vibronic bands

The dispersed fluorescence spectra following the excitation of several A<--X vibronic bands of HCCl and DCCl at visible wavelengths were successfully acquired in a discharge supersonic free jet expansion using an intensified charge-coupled device detector. The dispersed fluorescence spectra reveal more details of the X(1) A(') state vibrational structure in these molecules than previous reports. Dispersed fluorescence spectra of all four isotopomers (HC(35)Cl, HC(37)Cl, DC(35)Cl, and DC(37)Cl) were obtained. These dispersed fluorescence spectra exhibit the vibrational structures up to approximately 6000 cm(-1) above the zero-point level and determine the vibrational structures of HC(37)Cl and DC(37)Cl. Complete vibrational parameters including fundamental frequencies, anharmonicities, and coupling constants were determined for the HCCl/DCCl X(1) A(') state. Furthermore, perturbations from the background triplet state a(3) A(") and emission to triplet state levels were observed in the spectra. The singlet-triplet energy gap from the zero-point level could be determined to be 2167 cm(-1) (6.20+/-0.05 kcal/mol) in HCCl and to be 2187 cm(-1) (6.25+/-0.05 kcal/mol) in DCCl. Additionally, some of the A<--X excitation spectrum are reported for HCCl and DCCl.     

A clue to the diffuse structure in ultraviolet spectra of the GeCl2 A-X transition

Geometries, harmonic vibrational frequencies, and relative electronic energies of the two low-lying electronic states of the GeCl(2) dimer have been calculated at the CIS(D) method with a cc-pVTZ basis set. Minima corresponding to three isomers on the ground-state potential energy surface have been characterized. The most stable dimer has a dissociation energy of 0.74 eV and has a trans-(GeCl(2))(2) structure. There is also a related, less stable, cis minimum. A third, C(i) symmetry, isomer has a binding energy of 0.31 eV. It is found that this C(i) isomer has substantial dipole transition strength to the first excited singlet state of the dimer with a vertical excitation energy of 3.33 eV. The transition energy (T(0)) between this C(i) isomer and the van der Waals complex on the singlet excited state is predicted to be 4.007 eV, or a 1104 cm(-1) blueshift with respect to that of the GeCl(2) A-X transition. This finding may explain the diffuse structure which has been observed in the ultraviolet laser-induced fluorescence spectra of GeCl(2).     

Dispersed fluorescence spectroscopy of the CBr2A1B1-X1A1 transition

Dispersed fluorescence spectra following the excitation of the CBr2A1B1-X1A1 2 and 2 bands at visible wavelengths were acquired in a discharge supersonic free jet expansion using an intensified charge-coupled device (ICCD) detector. The dispersed fluorescence spectra show signal-to-noise ratios of up to 60 and extend out to a maximum red shift frequency of 6000 cm(-1). Complete and detailed vibrational structure of the ground singlet state (X1A1) was observed. Vibrational parameters including fundamental frequencies, anharmonicities, and coupling constants were determined for the CBr2 X1A1 state. Additional vibrational structure starting at approximately 3650 cm(-1) was observed and this indicates the singlet-triplet energy gap to be >10 kcal mol(-1).     

The electronic spectrum of Si3 I: triplet D(3h) system

We report the measurement of a jet-cooled electronic spectrum of the silicon trimer. Si(3) was produced in a pulsed discharge of silane in argon, and the excitation spectrum examined in the 18 000-20 800 cm(-1) region. A combination of resonant two-color two-photon ionization (R2C2PI) time-of-flight mass spectroscopy, laser-induced fluorescence/dispersed fluorescence, and equation-of-motion coupled-cluster calculations have been used to establish that the observed spectrum is dominated by the 1(3)A(1)" - a? (3)A(2)' transition of the D(3h) isomer. The spectrum has an origin transition at 18,600 ± 4 cm(-1) and a short progression in the symmetric stretch with a frequency of ～445 cm(-1), in good agreement with a predicted vertical transition energy of 2.34 eV for excitation to the 1(3)A(1)" state, which has a calculated symmetric stretching frequency of 480 cm(-1). In addition, a ～505 cm(-1) ground state vibrational frequency determined from sequence bands and dispersed fluorescence is in agreement with an earlier zero-electron kinetic energy study of the lowest D(3h) state and with theory. A weaker, overlapping band system with a ～360 cm(-1) progression, observed in the same mass channel (m/z = 84) by R2C2PI but under different discharge conditions, is thought to be due to transitions from the (more complicated) singlet C(2v) ground state ((1)A(1)) state of Si(3). Evidence of emission to this latter state in the triplet dispersed fluorescence spectra suggests extensive mixing in the excited triplet and singlet manifolds. Prospects for further spectroscopic characterization of the singlet system and direct measurement of the energy separation between the lowest singlet and triplet states are discussed.     

The ground state energy levels and molecular structure of jet-cooled HGeCl and DGeCl from single vibronic level emission spectroscopy

Single vibronic level dispersed fluorescence spectra of jet-cooled HGeCl and DGeCl have been recorded by laser excitation of selected bands of the A 1A"-X 1A' electronic transition. Twenty-six ground state vibrational levels of HGeCl and 42 of DGeCl were measured, assigned, and fitted to standard anharmonicity expressions, which allowed all the harmonic frequencies to be determined for both isotopomers. A normal coordinate least squares analysis obtained by fitting the harmonic frequencies yielded reliable values for five of the six force constants. The ground state effective rotational constants and force field data were combined to calculate average (rz) and approximate equilibrium (re z) structures, with re z(GeH)=1.586(1) A, re z(GeCl)=2.171(2) A, and the bond angle fixed at our CCSD(T)/aug-cc-pVTZ ab initio value of 93.9 degrees . Comparisons show that the derived bond lengths are consistent with those of the appropriate diatomic molecules in their ground electronic states and the bond angle is similar to that of germylene (GeH2). A Franck-Condon simulation of the vibrational intensities in the 0(0) (0) band emission spectrum of HGeCl using ab initio force field data shows good agreement with experiment, lending credence to the vibrational analysis of the observed spectra.     

Ab initio calculations and Franck-Condon simulation of the absorption spectra of GeCl2 including anharmonicity.

Geometrical parameters, vibrational frequencies and relative electronic energies of the X1A1, ?3B1 and A1B1 states of GeCl2 have been calculated at the CCSD(T) and/or CASSCF/MRCI level with basis sets of up to aug-cc-pV5Z quality. Core electron correlation and relativistic contributions were also investigated. RCCSD(T)/ aug-cc-pVQZ potential energy functions (PEFs) of the X1A1 and ?3B, states, and a CASSCF/MRCl/aug-cc-pVQZ PEF of the A1B1 state of GeCl2 are reported. Anharmonic vibrational wavefunctions of these electronic states of GeCl2, obtained variationally using the computed PEFs, are employed to calculate the Franck-Condon factors (FCFs) of the ?-X and A-X transitions of GeCl2. Simulated absorption spectra of these transitions based on the computed FCFs are compared with the corresponding experimental laser-induced fluorescence (LIF) spectra of Karolczak et al. [J. Chem. Phys. 1993, 98, 60-70]. Excellent agreement is obtained between the simulated absorption spectrum and observed LIF spectrum of the ?-X transition of GeCl2, which confirms the molecular carrier, the electronic states involved and the vibrational assignments of the LIF spectrum. However, comparison between the simulated absorption spectrum and experimental LIF spectrum of the A-X transition of GeCl2 leads to a revision of vibrational assignments of the LIF spectrum and suggests that the X1A1 state of GeCl2 was prepared in the experimental work, with a non-Boltzmann vibrational population distribution. The X(0,0,1) level is populated over 4000 times more than expected from a Boltzmann distribution at 60 K, which is appropriate for the relative population of the other low-lying vibrational levels, such as the X(1,0,0) and X(0,1,0) levels.     

Intermolecular vibrations of fluorobenzene-Ar up to 130 cm(-1) in the ground electronic state

Sixteen intermolecular vibrational levels of the S(0) state of the fluorobenzene-Ar van der Waals complex have been observed using dispersed fluorescence. The levels range up to ～130 cm(-1) in vibrational energy. The vibrational energies have been modelled using a complete set of harmonic and quartic anharmonic constants and a cubic anharmonic coupling between the stretch and long axis bend overtone that becomes near ubiquitous at higher energies. The constants predict the observed band positions with a root mean square deviation of 0.04 cm(-1). The set of vibrational levels predicted by the constants, which includes unobserved bands, has been compared with the predictions of ab initio calculations, which include all vibrational levels up to 70-75 cm(-1). There are small differences in energy, particularly above 60 cm(-1), however, the main differences are in the assignments and are largely due to the limitations of assigning the ab initio wavefunctions to a simple stretch, bend, or combination when the states are mixed by the cubic anharmonic coupling. The availability of these experimental data presents an opportunity to extend ab initio calculations to higher vibrational energies to provide an assessment of the accuracy of the calculated potential surface away from the minimum. The intermolecular modes of the fluorobenzene-Ar(2) trimer complex have also been investigated by dispersed fluorescence. The dominant structure is a pair of bands with a ～35 cm(-1) displacement from the origin band. Based on the set of vibrational modes calculated from the fluorobenzene-Ar frequencies, they are assigned to a Fermi resonance between the symmetric stretch and symmetric short axis bend overtone. The analysis of this resonance provides a measurement of the coupling strength between the stretch and short axis bend overtone in the dimer, an interaction that is not directly observed. The coupling matrix elements determined for the fluorobenzene-Ar stretch-long axis bend overtone and stretch-short axis bend overtone couplings are remarkably similar (3.8 cm(-1) cf. 3.2 cm(-1)). Several weak features seen in the fluorobenzene-Ar(2) spectrum have also been assigned.     

Laser-induced fluorescence spectroscopy of NC3S

In a discharged supersonic jet of acetonitrile and carbon disulfide, we have for the first time observed an electronic transition of the NC(3)S radical using laser-induced fluorescence (LIF) spectroscopy. A progression originating from the C-S stretching mode of the upper electronic state appears in the excitation spectrum. Each band of the progression has a polyad structure due to anharmonic resonances with even overtones of bending modes. Rotationally resolved spectra have been observed by high-resolution laser scans, and the electronic transition is assigned to A 2Pii-X 2Pii. For the vibronic origin band, the position and the effective rotational constant of the upper level have been determined to be 21 553.874(1) and 0.046 689(4) cm(-1), respectively. The dispersed fluorescence spectrum from the zero vibrational level of A 2Pi3/2 has also been observed; its vibrational structure is similar to that of the LIF excitation spectrum, showing a prominent C-S stretching progression with polyad structures. The vibrational frequencies of the C-S stretching mode in the ground and excited electronic states are determined to be 550 and 520 cm(-1), respectively. Fluorescence decay profiles have been measured for several vibronic levels of the A state.     

Laser induced fluorescence spectroscopy of a mixed dimer between 2-pyridone and 7-azaindole

We report here the laser induced fluorescence excitation (FE) and dispersed fluorescence (DF) spectra of a 1:1 mixed dimer between 7-azaindole (7AI) and 2-pyridone (2PY) measured in a supersonic free jet expansion of helium. Density functional theoretical calculation at the B3LYP/6-311++G** level has been performed for predictions of the dimer geometry and normal mode vibrational frequencies in the ground electronic state. A planar doubly hydrogen-bonded structure has been predicted to be the most preferred geometry of the dimer. In the FE spectrum, sharp vibronic bands are observed only for excitation of the 2PY moiety. A large number of low-frequency vibronic bands show up in both the FE and DF spectra, and those bands have been assigned to in-plane hydrogen bond vibrations of the dimer. Spectral analyses reveal Duschinsky-type mixing among those modes in the excited state. No distinct vibronic band structure in the FE spectrum was observed corresponding to excitations of the 7AI moiety, and the observation has been explained in terms of nonradiative electronic relaxation routes involving the 2PY moiety.     

Two-dimensional fluorescence (excitation/emission) spectroscopy as a probe of complex chemical environments

We report a new application of fluorescence spectroscopy for the identification and characterization of chemical species in complex environments. Simultaneous collection of a dispersed fluorescence spectrum for every step of the laser wavelength results in a two-dimensional spectrum of emission versus excitation wavelengths. This two-dimensional fluorescence (2DF) spectrum yields quick and intuitive assignments of a multitude of peaks in the separate fluorescence excitation and dispersed fluorescence spectra as belonging to the same species. We demonstrate the technique with the measurement of 2DF spectra of a discharge of dilute benzene into a supersonic free jet. A multitude of rovibronic bands due to the C(2) Swan and C(3) comet bands are immediately apparent and even unreported bands can be assigned intuituvely. Custom software filters are employed to enhance or reject emission from one or the other carrier to obtain excitation spectra arising from purely one carrier, or even a specific spectral component of a single carrier. The very characteristic 2DF fingerprints of C(2) and C(3) permit identification of another unidentified species in the discharge that absorbs at 476 nm, coincident with one of the diffuse interstellar bands.     

Fluorescence spectroscopy of jet-cooled chiral (+/-)-indan-1-ol and its cluster with (+/-)-methyl- and ethyl-lactate

The laser-induced fluorescence excitation, dispersed fluorescence, and IR-UV double resonance spectra of chiral (+/-)-indan-1-ol have been measured in a supersonic expansion. Three low energy conformers of the molecule have been identified, and the ground state vibrational modes of each conformer are tentatively assigned with the aid of quantum chemistry calculations. The frequencies of the nu(OH) and nu(CH) stretch modes of the two most abundant conformers have been measured by fluorescence dip IR spectroscopy and have been used for their assignment. The dispersed fluorescence spectra clearly indicate the coupling of low-frequency modes, as was seen in other substituted indanes such as 1-aminoindan and 1-amino-2-indanol. (R)- and (S)-indan-1-ol distinctly form different types of clusters with (R)- and (S)- methyl- and ethyl-lactate. Both hetero- and homochiral clusters are characterized by complex spectra which exhibit a progression built on low-frequency intermolecular modes.     

The ultraviolet spectrum of the CoCl2 radical, studied at vibrational and rotational resolution

The laser excitation spectrum of the 327 nm band system of CoCl2, formed in a free-jet expansion, has been recorded at a rotational temperature of approximately 10 K. The spectrum is congested and suffers extensive perturbations. A progression in the excited state symmetric stretching vibration has been identified. The decrease in the symmetric stretching vibrational wave number on excitation is considerable [nu1 '=195.7(12), nu1 (")=358.1(17) cm(-1)]. Despite widespread perturbations in the rotational structure of these vibronic bands, they can be confidently assigned to a parallel Omega=72-72 transition, consistent with an inverted 4Deltag ground electronic state. The rotational constant for Co35Cl2 in the ground state is determined to be 0.056 65(11) cm(-1), which corresponds to a value for the zero-point averaged Co-Cl bond length r0 of 2.062 8(40) A. The perturbations are found to be strongly isotopomer dependent.     

Conformations of 2-aminoindan in a supersonic jet: the role of intramolecular N-H...pi hydrogen bonding

Laser-induced fluorescence (LIF), dispersed fluorescence (DF), mass-resolved one-color resonance enhanced two-photon ionization (RE2PI) and UV-UV hole-burning spectra of 2-aminoindan (2-AI) were measured in a supersonic jet. The hole-burning spectra demonstrated that the congested vibronic structures observed in the LIF excitation spectrum were responsible for three conformers of 2-AI. The origins of the conformers were observed at 36931, 36934, and 36955 cm(-1). The DF spectra obtained by exciting the band origins of the three conformers showed quite similar vibrational structures, with the exception of the bands around 600-900 cm(-1). The molecular structures of the three conformers were assigned with the aid of ab initio calculations at the MP2/6-311+G(d,p) level. An amino hydrogen of the most stable conformer points toward the benzene ring. The stability of the most stable conformer was attributed to an intramolecular N-H...pi hydrogen bonding between the hydrogen atom and the pi-electron of the benzene ring. The other two conformers, devoid of intramolecular hydrogen bonding, were also identified for 2-AI. This suggests weak hydrogen bonding in the most stable conformer. The intramolecular N-H...pi hydrogen bonding in 2-AI was discussed in comparison with other weak hydrogen-bonding systems.     

Ultraviolet absorption and vibrational spectra of 2-fluoro-5-bromopyridine

The ultraviolet absorption spectrum in the range 340-185 nm in the vapour and solution phase has been measured for 2-fluoro-5-bromopyridine. Three fairly intense band systems identified as the pi* <-- pi transitions II, III and IV have been observed. A detailed vibronic analysis of the vapor and solution spectra is presented. The first system of bands is resolved into about sixty-two distinct vibronic bands in the vapour-phase spectrum. The 0,0 band is located at 35944 cm(-1). Two well-developed progressions, in which the excited state frequencies nu'25 (283 cm(-1)) and nu'19 (550 cm(-1)) are excited by several quanta, have been observed. The corresponding excited state vibrational and anharmonicity constants are found to be omega'i = 292 cm(-1), x'ii = 4.5 cm(-1) (i = 25) and omega'i = 563.8 cm(-1), x'ii = 6.9 cm(-1) (i = 19). The other two band systems show no vibronic structure, the band maxima being located at 48346 and 52701 cm(-1), respectively. The oscillator strength of the band systems in different solutions and the excited state dipole moments associated with the first two transitions have been determined by the solvent-shift method. The infrared spectrum in the region 4000-130 cm(-1) and the laser Raman spectrum of the molecule in the liquid state have been measured and a complete vibrational assignment of the observed frequencies is given. A correlation of the ground and excited state fundamental frequencies observed in the UV absorption spectrum with the Raman or infrared frequencies is presented.     

The intramolecular vibrational energy redistribution threshold in S(1) deuterated p-difluorobenzene

The intramolecular vibrational energy redistribution (IVR) in S(1) deuterated p-difluorobenzene (pDFB-d(4) or -d(4)) has been studied to determine the IVR threshold. For this, the S(1) <-- S(0) fluorescence excitation (FE) spectrum of jet-cooled d(4) was investigated in the 2000-3250 cm(-1) vibronic energy range of the S(1) electronic state, and single vibronic level fluorescence (SVLF) spectra have been acquired by exciting selected levels lying between 750 and 2850 cm(-1) in vibrational energy in the S(1) excited state. Congestion of the dispersed fluorescence in this molecule first appears as the vibrational level energy climbs above 2000 cm(-1). By comparing the SVLF spectra of pDFB-d(4) with those of p-difluorobenzene (pDFB or -h(4)), it is obvious that IVR threshold in -d(4) is localized with a few hundreds cm(-1) lower than that in pDFB. This decrease is entirely due to the increase in vibrational state density due to deuteration.     

Vibronic structure in the S1-S0 transition of jet-cooled dibenzofuran

Fluorescence excitation spectra of dibenzofuran in a supersonic jet are observed and the vibronic structure is analyzed for the S(1) (1)A(1) (pipi) and S(0) states. An observation of the rotational envelopes reveals that the band is a B-type band. However, it is shown that most of the strong vibronic bands are A-type bands. The intensity arises from vibronic coupling with the S(2) (1)B(2) state. We find a broad emission in the dispersed fluorescence spectrum for the excitation of the high vibrational levels in the S(1) state. This indicates that intramolecular vibrational redistribution (IVR) occurs efficiently in the isolated dibenzofuran molecule.     

Laser-induced fluorescence of cyclohexadienyl (c-C6H7) radical in the gas phase

A laser-induced fluorescence spectrum was observed in the 500-560 nm region when a mixture of 1,4-cyclohexadiene and oxalyl chloride was photolyzed at 193 nm. The observed excitation spectrum was assigned to the A (2)A(2)<--X (2)B(1) transition of the cyclohexadienyl radical c-C6H7, produced by abstraction of a hydrogen atom from 1,4-cyclohexadiene by Cl atoms. The origin of the A<--X transition of c-C(6)H(7) was at 18 207 cm(-1). From measurements of the dispersed fluorescence spectra and ab initio calculations, the frequencies of several vibrational modes in both the ground and excited states of c-C(6)H(7) were determined: nu(5)(C-H in-plane bend)=1571, nu(8)(C-H in-plane bend)=1174, nu(10)(C-C-C in-plane bend)=981, nu(12)(C-C-C in-plane bend)=559, nu(16)(C-C-C out-of-plane bend)=375, and nu(33)(C-C-C in-plane bend)=600 cm(-1) for the ground state and nu(8)=1118, nu(10)=967, nu(12)=502, nu(16)=172, and nu(33)=536 cm(-1) for the excited states.     

Reaction dynamics and vibrational spectroscopy of CH3D molecules with both C-H and C-D stretches excited

State-resolved reactions of CH3D molecules containing both C-H and C-D stretching excitation with Cl atoms provide new vibrational spectroscopy and probe the consumption and disposal of vibrational energy in the reactions. The vibrational action spectra have three different components, the combination of the C-H symmetric stretch and the C-D stretch (nu1 + nu2), the combination of the C-D stretch and the C-H antisymmetric stretch (nu2 + nu4), and the combination of the C-D stretch and the first overtone of the CH3 bend (nu2 + 2nu5). The simulation for the previously unanalyzed (nu2 + nu4) state yields a band center of nu0 = 5215.3 cm(-1), rotational constants of A = 5.223 cm(-1) and B = 3.803 cm(-1), and a Coriolis coupling constant of zeta = 0.084. The reaction dynamics largely follow a spectator picture in which the surviving bond retains its initial vibrational excitation. In at least 80% of the reactive encounters of vibrationally excited CH3D with Cl, cleavage of the C-H bond produces CH2D radicals with an excited C-D stretch, and cleavage of the C-D bond produces CH3 radicals with an excited C-H stretch. Deviations from the spectator picture seem to reflect mixing in the initially prepared eigenstates and, possibly, collisional coupling during the reaction.     

Photophysics and photochemical dynamics of chlorotoluene molecule

Laser-induced fluorescence spectra of jet-cooled chlorotoluene molecules are reported for the S₁ state. The fluorescence excitation spectrum of m-chlorotoluene shows some low-frequency bands up to 200 cm⁻¹ above the S₁ origin, which are assigned to internal rotational modes of the methyl group. Beyond 300 cm⁻¹ and up to approximately 1500 cm⁻¹ sharp vibrational bands are observed, which are assigned by measurement of the dispersed fluorescence spectrum on excitation of each vibrational band. The vibrational energies of the C---Cl stretching modes for the o-, m- and p-chlorotoluene molecules are 341, 378 and 360 cm⁻¹ respectively in the S₁ state.     

Dispersed fluorescence spectroscopy of primary and secondary alkoxy radicals

Dispersed fluorescence (DF) spectra of 1-propoxy, 1-butoxy, 2-propoxy, and 2-butoxy radicals have been observed under supersonic jet cooling conditions by pumping different vibronic bands of the B-X laser induced fluorescence excitation spectrum. The DF spectra were recorded for both conformers of 1-propoxy, three conformers of the possible five of 1-butoxy, the one possible conformer of 2-propoxy, and two conformers of the possible three of 2-butoxy. Analysis of the spectra yields the energy separations of the vibrationless levels of the ground X and low-lying A electronic state as well as their vibrational frequencies. In all cases, the vibrational structure of the DF spectra is dominated by a CO stretch progression yielding the nuCO stretching frequency for the X state and in most cases for the A state. In addition to the experimental work, quantum chemical calculations were carried out to aid the assignment of the vibrational levels of the X state and for some conformers the A state as well. Geometry optimizations of the different conformers of the isomers were performed and their energy differences in the ground states were determined. The results of the calculation of the energy separations of the close-lying X and A states of the different conformations are provided for comparison with the experimental observations.