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Optical frequency comb-referenced measurements of self pressure-broadened line profiles of the R(8) to R(13) lines in the ν1 + ν3 combination band of acetylene near 1.52 µm are reported. The analysis of the data found no evidence for a previously reported [Iwakuni et al., Phys. Rev. Lett. 117(14), 143902 (2016)] systematic alternation in self pressure-broadened line widths with the nuclear spin state of the molecule. The present work brought out the need for the use of an accurate line profile model and careful accounting for weak background absorptions due to hot band and lower abundance isotopomer lines. The data were adequately fit using the quadratic speed-dependent Voigt profile model, neglecting the small speed-dependent shift. Parameters describing the most probable and speed-dependent pressure-broadening, most probable shift, and the line strength were determined for each line. Detailed modeling of the results of Iwakuni et al. showed that their neglect of collisional narrowing due to the speed-dependent broadening term combined with the strongly absorbing data recorded and analyzed in transmission mode were the reasons for their results. [ABSTRACT FROM AUTHOR]

Frequency comb-referenced measurements of sub-Doppler laser saturation dip absorption lines in the v 1 + v 3 band of acetylene near 1.5 μm are reported. These measurements include transitions involving higher rotational levels than previously frequency measured in this band. The accuracy of the measured frequencies is typically better than 10 kHz. Measurements of the observed sub-Doppler line widths as a function of pressure showed that the self-pressure-broadening coefficients are about 3.5 times larger than those derived from conventional pressure broadening of unsaturated Doppler-limited spectra. This is attributed to the contribution of velocity-changing collisions to the total dephasing rate in the low pressure sub-Doppler measurements. At higher pressures, when the homogeneous broadening becomes comparable to the typical Doppler shift per elastic collision, the velocity changing collisions cease to contribute significantly to the incremental pressure broadening. A time-dependent soft collision model is developed to illustrate the transition between low and high pressure regimes of sub-Doppler pressure-broadening. [ABSTRACT FROM AUTHOR]

Sub-Doppler, saturation dip, spectra of lines in the v1 + v3, v1 + 2v4, and v3 + 2v4 bands of 14NH3 have been measured by frequency comb-referenced diode laser absorption spectroscopy. The observed spectral line widths are dominated by transit time broadening and show resolved or partially-resolved hyperfine splittings that are primarily determined by the 14N quadrupole coupling. Modeling of the observed line shapes based on the known hyperfine level structure of the ground state of the molecule shows that, in nearly all cases, the excited state level has hyperfine splittings similar to the same rotational level in the ground state. The data provide accurate frequencies for the line positions and easily separate lines overlapped in Doppler-limited spectra. The observed hyperfine splittings can be used to make and confirm rotational assignments and ground state combination differences obtained from the measured frequencies are comparable in accuracy to those obtained from conventional microwave spectroscopy. Several of the measured transitions do not show the quadrupole hyperfine splittings expected based on their existing rotational assignments. Either the assignments are incorrect or the upper levels involved are perturbed in a way that affects the nuclear hyperfine structure. [ABSTRACT FROM AUTHOR]

Transient diode laser absorption spectroscopy has been used to measure three strong vibronic bands in the near infrared spectrum of the C2H, ethynyl, radical not previously observed in the gas phase. The radical was produced by ultraviolet excimer laser photolysis of either acetylene or (1,1,1)-trifluoropropyne in a slowly flowing sample of the precursor diluted in inert gas, and the spectral resolution was Doppler-limited. The character of the upper states was determined from the rotational and fine structure in the observed spectra and assigned by measurement of ground state rotational combination differences. The upper states include a ²Σ+ state at 6696 cm-1, a second ²Σ+ state at 7088 cm-1, and a ²Π state at 7110 cm-1. By comparison with published calculations [R. Tarroni and S. Carter, J. Chem. Phys 119, 12878 (2003); Mol. Phys. 102, 2167 (2004)], the vibronic character of these levels was also assigned. The observed states contain both X ²Σ+ and A 2? electronic characters. Several local rotational level perturbations were observed in the excited states. Kinetic measurements of the time-evolution of the ground state populations following collisional relaxation and reactive loss of the radicals formed in a hot, non-thermal, population distribution were made using some of the strong rotational lines observed. The case of C2H may be a good place to investigate the behavior at intermediate pressures of inert colliders, where the competition between relaxation and reaction can be tuned and observed to compare with master equation models, rather than deliberately suppressed to measure thermal rate constants. [ABSTRACT FROM AUTHOR]

Singly deuterated silylene has been detected and characterized in the gas-phase using high-resolution, two-dimensional, optical spectroscopy. Rotationally resolved lines in the 000X¹A' → A¹A″ band are assigned to both c-type perpendicular transition and additional parallel, axis-switching induced bands. The extracted rotational constants were combined with those for SiH2 and SiD2 to determine an improved equilibrium bond length, rSiH, and bond angle, θ, of 1.5137 ± 0.0003 Å and 92.04? ± 0.05?, and 1.4853 ± 0.0005 Å and 122.48? ± 0.08? for the X¹A' (0,0,0) and A¹A″(0,0,0) state respectively. The dispersed fluorescence consists of a long progression in the A¹A″(0,0,0) →X¹A'(0,ν2,0) emission which was analyzed to produce vibrational parameters. A strong quantum level dependence of the rotationally resolved radiative decay curves is analyzed. [ABSTRACT FROM AUTHOR]

Naphthalene, anthracene, and phenanthrene are shown to have very long-lived triplet lifetimes when the isolated molecules are excited with nanosecond pulsed lasers resonant with the lowest singlet state. For naphthalene, triplet state populations are created only during the laser pulse, excluding the possibility of normal intersystem crossing at the one photon level, and all molecules have triplet lifetimes greater than hundreds of microseconds, similar to the behavior previously reported for phenylacetylene. Although containing 7-12 thousand cm-1 of vibrational energy, the triplet molecules have ionization thresholds appropriate to vibrationless T1 states. The laser power dependences (slopes of log-log power plots) of the excited singlet and triplet populations are about 0.7 for naphthalene and about 0.5 for anthracene. Kinetic modeling of the power dependences successfully reproduces the experimental results and suggests that the triplet formation mechanism involves an enhanced spin orbit coupling caused by sigma character in states at the 2-photon level. Symmetry adapted cluster-configuration interaction calculations produced excited state absorption spectra to provide guidance for estimating kinetic rates and the sigma character present in higher electronic states. It is concluded that higher excited state populations are significant when larger molecules are excited with pulsed lasers and need to be taken into account whenever discussing the molecular photodynamics. [ABSTRACT FROM AUTHOR]

Perturbations in the 716 and 818 mixed singlet/triplet levels of ã 1A1(0,0,0) methylene, CH2, have been reinvestigated by frequency-modulated laser sub-Doppler saturation spectroscopy. The hyperfine structure was completely resolved for both the predominantly singlet and the predominantly triplet components of these mixed rotational levels using b 1B1-ã 1A1 optical transitions near 12 200 cm-1 with megahertz resolution. The mixing coefficients were obtained from the observed hyperfine splittings and a two-level deperturbation model. The analysis also determines the energy separation of the unperturbed zero-order levels and the unperturbed hyperfine splittings for the triplet perturbing levels 615 X 3B1(0,3,0) and 937 X 3B1(0,2,0). [ABSTRACT FROM AUTHOR]

A method is described for the use of electronic structure and Franck-Condon factor programs in the calculation of the vibrational intensities in forbidden electronic transitions. Using the B 2B2-X 2B1 electronic transition of benzonitrile cation as a test case, transition moments were calculated using the symmetry adapted cluster/configuration interaction method at various points along the normal mode displacements of the molecule, from which transition moment derivatives were obtained. The transition moments were found to vary almost linearly with respect to the normal mode displacements. Using these, along with Franck-Condon factors, an expansion of the transition moment with respect to the normal coordinates provides a measure of vibrational intensities, including the effects of geometry change and Duschinsky rotation [Acta Physicochim. URSS 7, 551 (1937)]. Second order terms in the moment expansion are calculated, and it is determined that they must be included if the intensity of combination bands is to be properly obtained. [ABSTRACT FROM AUTHOR]

Photoinduced Rydberg ionization (PIRI) spectra of the second excited electronic state of benzonitrile cation were recorded via the origin and 6a1 and 6b1 vibrational levels of the cation ground electronic state. This B←X transition was verified to be a forbidden 2B2←2B1 transition with an origin at 17 225 cm-1 above the ground ionic state. By the use of vibronic coupling calculations, as well as symmetry analysis and comparison of the PIRI spectra via different ground vibrational levels, a nearly complete assignment of the vibrational structure was made, and the vibrational frequencies of the B 2B2 state of benzonitrile cation were obtained based on the assignments. Comparisons of the experimental spectra with simulations from the vibronic structure calculations are also used to validate the theoretical procedures used in the simulations. [ABSTRACT FROM AUTHOR]

Geometries, harmonic vibrational frequencies, and relative electronic energies of the two low-lying electronic states of the GeCl2 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-(GeCl2)2 structure. There is also a related, less stable, cis minimum. A third, Ci symmetry, isomer has a binding energy of 0.31 eV. It is found that this Ci 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 (T0) between this Ci 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 GeCl2 A-X transition. This finding may explain the diffuse structure which has been observed in the ultraviolet laser-induced fluorescence spectra of GeCl2. [ABSTRACT FROM AUTHOR]

The role of mixed states in the collision-induced thermalization, intersystem crossing, and reactive loss of CH2 (ã 1A1) has been monitored using Doppler-resolved transient frequency modulation absorption spectroscopy. Singlet CH2 is produced in a hot initial distribution of translation and rotational energy states in the 308 nm photodissociation of ketene in a large excess of argon. Collisions with Ar and ketene cool the translational and rotational degrees of freedom, while depleting the total singlet CH2 population through reaction and intersystem crossing. Direct monitoring of the time-dependent populations of rotational levels containing mixed singlet and triplet character reveals a rapid interconversion between the two components, but no discernable difference between the kinetics of the pure singlet and mixed states at longer times. [ABSTRACT FROM AUTHOR]

The spectrum of methylene in the 1.3–1.4 and 0.89–0.94 μm wavelength regions has been recorded in absorption using frequency-modulated cw diode and Ti:sapphire laser sources. The spectral lines have Doppler-limited resolution and have been assigned to bands in the b 1B1←ã 1A1 electronic spectrum of the radical. In three of the four bands studied, the lower state is the bend excited, υ2″=1, level of the ã state and two of the upper levels lie below the energy of the degenerate linear configuration of the b/ã pair. Together with previously measured data pertaining to υ2″=1, the data have been used to refine the precision of the experimentally determined rotational structure in this level. Although several K″=1 levels do show shifts of more than 0.1–0.2 cm-1, multiple strong perturbations due to near-resonant background X 3B1 rovibrational levels, such as are known to occur in the ã 1A1, υ2″=0 level have not been found in υ2″=1. Absorption lines due to the predominantly triplet X(040) 414 level, responsible for most of the perturbation of ã(010) 515, have been identified in the spectra. The data also fix the energies of the b(0,0,0)2, ã(0,7,0)1, b(0,2,0)3, and ã(0,10,0)2 upper vibronic levels, where the numbers in parentheses are the vibrational quantum numbers with superscript K, the projection of the total angular momentum on the a-inertial axis. [ABSTRACT FROM AUTHOR]

Rotationally resolved spectra of several bands lying to the red of the origin of the ùA"-...A' band system of chloromethylene (HCCl), were recorded by laser absorption spectroscopy in ambient temperature and jet-cooled samples. The radical was made by excimer laser photolysis of dibromochloromethane, diluted in inert gas, at 193 nm. The jet-cooled sample showed efficient rotational but less vibrational cooling. Analysis showed that the observed bands originate in the (υ1, υ2, υ3)=(010), (001), and (011) vibrational levels of the ground electronic state of the radical, while the upper-state levels involved were (000), (010), (001), and (011). Vibrational energies and rotational constants describing the rotational levels in the lower-state vibrational levels were determined by fitting to combination differences. The analysis also resulted in a reevaluation of the C-Cl stretching frequency in the excited state and we find E'001=13 206.57 or 926.17 cm-1 above the ÃA"(000) rotationless level for HC35Cl. Scaled ab initio potential energy surfaces for the à and ... states were used to compute the transition moment surface and thereby the relative intensities of different vibronic transitions, providing additional support for the assignments and permitting the prediction of the shorter wavelength spectrum. All the observed upper state levels showed some degree of perturbation in their rotational energy levels, particularly in Ka=1, presumably due to coupling with near-resonant vibrationally excited levels of the ground electronic state. Transitions originating in the low-lying ã³A" were also predicted to occur in the same wavelength region, but could not be identified in the spectra. [ABSTRACT FROM AUTHOR]

We report the observation of the rotationally resolved spectrum of the c 1A1 state of CH2 via sequential single-photon absorptions at visible and near-infrared wavelengths. Direct absorption from the lowest singlet state ã 1A1 to c 1A1 occurs in the near UV, but it is weak because it corresponds to a two electron transition between the dominant single configuration approximations to the electronic wave functions. Some absorption lines in the c-ã system were originally reported in 1966 [G. Herzberg and J. W. C. Johns, Proc. R. Soc. London, Ser. A 295, 107 (1966)], but the weak spectra could not be assigned at the time. Interest in the c 1A1 state was rekindled by recent ab initio results [S. N. Yurchenko, P. Jensen, Y. Li, R. J. Buenker, and P. R. Bunker, J. Mol. Spectrosc. 208, 136 (2001)] which prompted the present work. The new spectra provide accurate energies for rotational levels in the v2linear=11,l=1 level of the state, and permit assignment of most of the line positions measured by Herzberg and Johns. The double-resonance technique used may be easily extended to the measurement of lower rovibrational levels in the electronic state and possibly also to access the d 1A2 state which is theoretically expected to lie at similar energies but, for symmetry reasons, is not accessible from the lowest singlet state in a single electric-dipole transition. [ABSTRACT FROM AUTHOR]

The potential energy surface for methyl cation is calculated using an extrapolated full coupled-cluster/complete basis set (FCC/CBS) theory. The equilibrium C-H bond length is obtained as 1.0884 Å. The vibrational energy levels of both CH[sup +,sub 3] and CD[sup +,sub 3] cations are computed using a variational method, based on the accurate potential energy surface. The fundamental frequencies in cm[sup -1] are predicted to be 2942.27 (υ[sub 1]), 1377.82 (υ[sub 2]), 3108.29 (υ[sub 3]), and 1387.01 (υ[sub 4]) for CH[sup +,sub 3], D and 2082.99 (υ[sub 1]) 1070.31 (υ[sub 2]) 2339.99 (υ[sub 3]), and 1015.70 (υ[sub 4]) for C D[sup +,sub 3] , respectively. [ABSTRACT FROM AUTHOR]

A K-dependent adiabatic approach is suggested to study the Renner–Teller effect in triatomic molecules using a two-dimensional Hilbert space approximation in the electronic degree of freedom. The theory is developed in hyperspherical coordinates, and approximately includes the electronic spin–orbit interaction. The adiabatic Hamiltonians are expressed in terms of a K-dependent effective potential energy surface. Eigenpairs are calculated by solving the time-independent Schro¨dinger equation, which is represented in a mixed grid/basis set. The method is applied to the A˜←X˜ band system of bromomethylene (H/DCBr). The results obtained show that the Renner–Teller effect in this molecule is pronounced, particularly in the excited A˜ [sup 1]A[sup ″] state. The results are compared to recent experimental measurements and good agreement is found. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Describes the interaction between molecules of ethyl radical with small torsional barrier. Range of action between internal rotation and other normal modes of the molecule; Use of electronic structure calculations to estimate both the electronic and vibrational contributions to the barrier.

Reports the infrared spectrum measurements of ethyl radical carbon hydrogen[sub 2] out-of-plane rocking vibrational fundamental by transient diode laser absorption spectroscopy. Data providing information on the structure of and the barrier of internal rotation; Examination of how quantities change on vibrational excitation.

The rotationally resolved spectrum of the HCBr A 1A″(0,2,0)←X 1A′(0,0,0) Ka=0←1 transition between 12760 and 12850 cm-1 was obtained for the first time at Doppler-limited resolution using a transient frequency-modulation absorption technique. Rotational structure of HC 79Br and HC 81Br was identified and analyzed. The analysis shows R″(C–Br)=1.852 Å and R′(C–Br)=1.749 Å. The observed band indicates a linear–bent transition. This yields an upper limit of approximately 1600 cm-1 for the barrier to linearity above the zero-point energy for the A 1A″ state. Perturbations caused by singlet–triplet interactions were also found in the observed spectrum. The analysis of these perturbations indicates a very low-lying a 3A″ state. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The observation and analysis of the high resolution spectrum of the CH2 rocking fundamental of the ethyl radical, C2H5, at wavelengths close to 18.9 μm is reported. The band origin is found to be at 528.1 cm-1. The spectrum shows evidence for a very low barrier to internal rotation, or torsion, in this species. A simple model Hamiltonian, based on an assumed structure with G12 symmetry, qualitatively reproduces the observations and implies a torsional barrier of approximately 20 cm-1 in both the zero point and excited vibrational states. The Hamiltonian cannot reproduce the observations to the level of the experimental accuracy and the importance of several neglected terms is tested and discussed. The observed torsional splittings imply that, within the confines of the model, the a-inertial and internal rotation axes in the molecule are coincident to within 1°. In addition to their intrinsic interest, the spectra will be useful for future state resolved studies of the kinetics of chemical reactions involving the radical. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The rotationally resolved spectrum of the HCCl Ã1A″(0,0,0)←X1A′(0,0,0) transition between 12 230 and 12 410 cm-1 was obtained for the first time at Doppler-limited resolution using a frequency-modulation (FM) transient absorption technique. Five major subbands (Ka = 0←1, 1←0, 1←2, 2←1, and 2←3) of HC35Cl and one subband (Ka = 0←1) of HC37Cl were observed and assigned. In addition, the forbidden subband (Ka = 0←0) of HC35Cl was also observed. Thus far, we have assigned most (more than 70%) of the strong lines. The analysis of these subbands determines the geometry for the HCCl Ã(0,0,0) level: /_ HCCl = 134.7° and RC-Cl = 1.623 Å while RC-H is fixed at the corresponding value of CH2. The height of the barrier to linearity was found to be approximately 2170 cm-1 above the vibrationless level of the à state. The rotational structure of this spectrum shows irregular perturbations. Both random and J-dependent (anomalous K-type doubling) effects were observed. As in methylene, the perturbations most likely arise from Renner-Teller effect between the two singlet states in combination with spin–orbit coupling between the singlet and triplet states. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Far infrared laser magnetic resonance spectroscopy is used to measure components of 12 rotational transitions in the ground state of the HOCO radical. The transitions are all b-dipole in character in contrast to the a-dipole rotational spectrum previously reported [Radford, Wei, and Sears, J. Chem. Phys. 97, 3989 (1992)]. The new data determine the A rotational constant to high precision and allow the determination of several centrifugal distortion constants for the first time. The hyperfine coupling in the radical leads to observable splittings in several of the observed transitions and these are used to estimate two of the four expected nonzero hyperfine parameters in the radical. [ABSTRACT FROM AUTHOR]

We report the observation and assignment of the ν2 fundamental vibration in the HOCO and DOCO radicals. The radical was made by the photolysis of acetic acid or acetic acid-d at 193 nm in a flow system. The observed spectra indicate that the excited vibrational level is perturbed in both HOCO and DOCO. In HOCO, the rotational levels in ν2=1 have an irregular dependence on the Ka quantum number, probably caused by anharmonic interactions with combinations of lower frequency in plane vibrations. An a-type Coriolis interaction involving perturbation by a level containing one quantum of the out-of-plane torsional vibration cannot be rigorously excluded however. In DOCO, there are also N-dependent perturbations in the excited state. Only parallel transitions were assigned in the ν2 fundamental of both isotopomers. The K=0 band origin for HOCO is at 1852.567 cm-1 which compares with 1843.7 cm-1 reported previously for the radical trapped in a low-temperature argon matrix. This absorption spectrum will be useful in future studies of chemical reactions involving the radical. [ABSTRACT FROM AUTHOR]

Photodissociation of the molecules RNCS and RSCN (R=H, CH3, and C2H5) was investigated at 248 and 193 nm and the internal energy distributions in the CN resulting from the RS+CN fragmentation channel were probed by laser induced fluorescence. These CN distributions were identical for formation from the isomer pairs in agreement with an excited state isomerization postulated earlier. At 248 nm, all precursors lead to nearly thermal CN rotational distributions with rotational temperatures of approximately 1100 K in v=0 and 800 K in v=1. The vibrational distributions could not be characterized by the same temperatures. At 193 nm, contributions from two apparently different dissociation channels were observed. The CN rotational populations showed a narrow, strongly peaked distribution lying on a broad, approximately statistical distribution. For HNCS, this peak occurs near N=10 suggesting dissociation from an excited state with a linear NCS skeleton. For the alkyl precursors, the peak occurs at very high rotational quantum number (N=70) indicating an excited state with a bent NCS framework. Vibrational distributions with population out to v=6 were observed. In all cases, the apparent statistical part of the rotational distribution and the vibrational distributions can be described by a prior function derived assuming that the internal modes of the alkyl substituent are not involved in any energy redistribution during the dissociation. [ABSTRACT FROM AUTHOR]

The 248 and 193 nm photodissociation of the series of molecules RNCS and RSCN (R=H, CH3, C2H5) has been investigated. Laser-induced fluorescence of the radical NCS was used to probe the energy deposition in the R+NCS channel of this dissociation, however excitation to many vibrational states of NCS caused spectral congestion which prohibited direct measurement of populations of individual levels. A single-photon dissociation channel leading to RS and CN was observed for both isomers of all of the precursor molecules. While this was expected for the thiocyanate species, no direct channel to these products is available for the isothiocyanates. This observation is discussed in terms of an excited state isomerization of these molecules. Measured ratios of CN to NCS production for all compounds provide additional support for this mechanism and suggest a significant barrier to the process. Comparison of nascent laser-induced fluorescence (LIF) spectra of NCS with spectra obtained following different amounts of collisional relaxation suggests that all three vibrational modes were equally excited with a vibrational temperature of roughly 4500 K. The rotational temperature was estimated to be less than 1000 K. A spin–orbit population inversion was observed for all vibrational levels with approximately 75% of the population in the upper spin–orbit component of the vibrationless and low lying bending excited levels. No simple model is found to explain the photodissociation dynamics or the isomerization. [ABSTRACT FROM AUTHOR]

The infrared absorption spectrum of the ν3 fundamental band of the CD3 radical has been detected by diode laser absorption spectroscopy. The CD3 radical was produced by excimer laser photolysis of CD3I at 248 nm or (CD3)2CO at 193 nm. Molecular parameters of the v3=1 vibrational state were determined from a least-squares fit to 62 rotation–vibration transitions. In this fit, molecular parameters describing the ground state were constrained to those obtained from previous spectroscopic studies of the ν2 parallel IR band [J. M. Frye, T. J. Sears, and D. Leitner, J. Chem. Phys. 88, 5300 (1988)]. The molecular parameters determined in the present work are the band origin ν0=2381.088 60(84), B’=4.758 737(40), C’=2.373 297(34), (ζC)3=0.476 278(72), q3=0.003 76(59), D’N =0.000 187 9(5), DNK =-0.000 341 0(12), D’K =0.000 143 7(8), ηN =-0.000 005 5(36), η’K =0.000 060(35), and qN =0.000 063(17), all in cm-1 with one standard deviation in parentheses. The derived molecular parameters were compared with those for the CH3 radical v3=1 level determined previously [T. Amano, P. Bernath, C. Yamada, Y. Endo, and E. Hirota, J. Chem. Phys. 77, 5284 (1982)]. The molecular parameters of the v3=1 state of the CD3 and CH3 radicals follow the expected isotopic relationships. We have also found that the determined molecular parameters reasonably satisfy the approximate planarity relationships [J. K. G. Watson, J. Mol. Spectrsoc. 65, 123 (1977)] and the sign of the l-type doubling constant is consistent with a planar equilibrium structure. [ABSTRACT FROM AUTHOR]

Diode laser transient absorption/gain spectroscopy is used to monitor time-dependent populations of high rotational levels in OD (v=0) produced in the reaction of O(1D)+D2. Pure rotational transitions on species with large dipole moments offer good sensitivity, full state resolution and μs time resolution in the present apparatus. Measured nascent populations of OD in the four highest rotational levels thermodynamically accessible in this reaction are in reasonable agreement with the reported results of earlier laser-induced-fluorescence measurements, in which corrections for transition moments and predissociation introduce increasing uncertainties at high rotational levels. The relaxation kinetics of the highest rotational levels are not hopelessly complex, and evidence is presented for strong, but not complete propensity for conservation of Λ doublet symmetry during rotational relaxation. [ABSTRACT FROM AUTHOR]

Laser-induced fluorescence excitation spectra have been recorded for many vibronic transitions of the A(2Π)–X(2Π) and B(2Σ+)–X(2Π) systems of NCS under supersonic free jet expansion conditions. New assignments have been made for many of these bands and several of the assignments from previous work have been revised. Vibronic energies have been determined for levels involving excitation of all three vibrational modes in the ground electronic state and both excited electronic states. A detailed rotational and vibrational analysis has been carried out for levels involving excitations of the two stretching modes and the many rotational and potential function constants have been determined for the A and X states. The variation of the spin–orbit constant with vibrational level has also been investigated. Results of this paper provide the ground work for a detailed analysis of the Renner–Teller effect in the bending vibrational mode of this radical for both the X2(Π) and A(2Π) states to appear in a forthcoming paper. They also provide a spectroscopic database for future dynamical and kinetics studies of processes involving this radical. [ABSTRACT FROM AUTHOR]

Transient diode laser absorption spectroscopy has been used to monitor the rotational and vibrational populations of CD3 radicals following photodissociation of CD3I by 248 nm light. Rotational lines in the ν2=1–0, 2–1, 3–2, and 4–3 bands have been analyzed to give time-dependent CD3 populations following photodissociation of 100 mTorr samples of neat CD3I. Prompt absorption signals are small, but positive for all observed rotational lines of all observed bands, providing clear evidence against population inversion in the low levels of the ν2 vibration of methyl photoproducts. The vibrational population distribution in ν2 a few μs after photodissociation is 4:3:2 for v=0:1:2 with about 20% uncertainties. A strong variation in time dependence with rotational level is consistent with an initially hot rotational distribution (≥2000 K) that cools on the μs time scale of methyl translational moderation in the bath of undissociated methyl iodide. We believe the origin of the rotational excitation to be translation–rotation energy transfer, rather than the photodissociation itself. Time-resolved Doppler line shapes provide an independent view of the translational energy loss. [ABSTRACT FROM AUTHOR]

Studies the v[sub 2] vibrational spectrum of deuterated methyl radical CD[sub 3]. Vibrational potential function for the species; Positions of unobserved vibrational bands for all symmetric isotopic modifications; Population distribution of the v[sub 2] levels.

CD3 radicals were generated by excimer laser photolysis (248 nm) of CD3 I, and the transient populations detected by diode laser absorption using a gated integrator signal averager. The v2 =1←0, 2←1, and 3←2 bands of the CD3 radical were observed, and band center frequencies were determined to be 457.8133(11), 507.9311(19), and 542.4510(29) cm-1, respectively. Observed transitions were fitted, by least squares analysis, to yield rotational and centrifugal distortion constants and spin–rotation parameters, and results are compared to theory. Measurements were also carried out of the time dependence of the vibration–rotation level populations as a function of vibrational excitation and spin component. [ABSTRACT FROM AUTHOR]

Five rotational transitions involving Ka=4←3 in the ground vibronic state of the methylene radical have been detected by diode laser absorption spectroscopy in the infrared at wavelengths around 25 μm. The radical was formed in a mild glow discharge in a dilute mixture of ketene in helium that was pumped continuously through the absorption cell, and the methylene absorption lines were detected using a Zeeman modulation technique. The solenoid used to generate the required oscillating magnetic field was powered by a simpler and less expensive electrical circuit than has previously been described for this type of spectroscopy. [ABSTRACT FROM AUTHOR]

Examines an unaccessed region of the near-infrared singlet...singlet absorption spectrum of methylene. Implications of the diode laser spectrometer for signal-to-nose ratios close to the quantum noise limit; Identification of rovibronic transitions of unobserved levels; Speculation on halomethylenes from spectroscopic observation.

We have simulated the photoelectron spectrum of CH-2 using the model described previously [Sears and Bunker, J. Chem. Phys. 79, 5265 (1983)]. The optimization of the fit of the simulated spectrum to the recently observed spectrum of Lineberger and co-workers [J. Chem. Phys. 81, 1048 (1984) and preceding paper] has enabled us to determine the rotation-bending energy levels of triplet CH2 over an energy range of more than 1 eV. It has also enabled us to determine that the rotational temperature of the CH-2 in the experiment is 220 K and that, for v2=1, the vibrational temperature is 680 K. For CH-2 we determine that ae=103° and that ν2=1230 cm-1. The singlet–triplet splitting in methylene is determined to be 3150±30 cm-1 (0.3905±0.004 eV, 9.01±0.09 kcal/mol) from the photoelectron spectrum, in excellent agreement with the more accurate value previously obtained from LMR spectroscopy [McKellar et al., J. Chem. Phys. 79, 5251 (1983)] of 3165±20 cm-1 (0.3924±0.0025 eV, 9.05±0.06 kcal/mol), and the electron affinity of triplet CH2 is determined to be 0.652±0.006 eV. [ABSTRACT FROM AUTHOR]

High resolution measurements of the two components of the ν2 (umbrella) fundamental mode of the fully deuterated oxonium ion (D3O+) are reported. The spectra were obtained by diode laser absorption in a plasma containing D2 and D2O. Analysis of the observed spectra yields precise estimates of the two band origins and molecular parameters describing the states involved. By combining these data with the available high resolution data for H3O+, using the nonrigid invertor Hamiltonian, we are able to determine the equilibrium structure and the vibrational potential function; also we predict the inversion spectrum of D3O+ and the 2v2–v2 hot band spectra of H3O+ and D3O+. [ABSTRACT FROM AUTHOR]

We report extensive measurements of rovibronic levels in the 020/100 and 040/120/200 Fermi-resonant polyads of NCO. The levels were accessed by the technique of stimulated emission pumping spectroscopy in a supersonic free jet expansion following excitation of bands in the A 2Σ+-X 2Π spectrum of the radical. The data were analyzed in terms of an effective Hamiltonian which explicitly included reference to all the possible vibronic levels of 2Π symmetry in the polyad under consideration. The effects of levels outside these were treated by perturbation theory. The model was successful in fitting the data close to the experimental measurement precision and the resulting parameters were interpreted in terms of the harmonic and anharmonic vibrational terms. The effective Renner–Teller coupling parameter in NCO varied with vibrational level; however, parameters describing the Fermi-resonance interaction were found to be constant for all the levels investigated. [ABSTRACT FROM AUTHOR]

Part of the rotational spectrum of the hydroxyformyl radical, HOCO, and its deuterated analog has been detected at frequencies between 230 and 300 GHz. The radical was formed in a flow system by the reaction between chlorine atoms and formic acid. Analysis of the spectra yields reliable estimates of the rotational, centrifugal distortion and spin–rotational parameters describing the ground state of this species. The rotational constants derived for HOCO and DOCO are consistent with a planar equilibrium structure and are used to confirm that the carrier of the spectrum is the trans-geometrical isomer. [ABSTRACT FROM AUTHOR]

Laser induced fluorescence measurements of NCO formed in the reaction CN+O2→NCO+O in a free jet expansion are reported. The rotational temperature achieved in the experiments was typically 10–15 K and absorption from vibrationally excited levels was very much reduced in intensity. Two bands in the A–X system, the v1, v2, v3=200–000 and 120–000, of the radical were rotationally resolved for the first time, and wavelength resolved laser excited fluorescence measurements have been used to characterize many ground state vibronic energy levels below 6000 cm-1 in energy. Stimulated emission pumping experiments were carried out on vibronic levels associated with the ν3 vibration with 1≤v3≤3 in the electronic ground state. Harmonic and anharmonic constants were determined, together with the spin–orbit coupling parameters and rotational constants for these levels. [ABSTRACT FROM AUTHOR]

Diode laser transient absorption/gain spectroscopy is used to monitor time-dependent populations of CD3 fragments formed in the photodissociation of acetone-d6 at 193 nm. Selected rotational lines have been measured in the ν2 ‘‘umbrella’’ fundamental and first two hot bands, and in the ν3 asymmetric stretching fundamental band. Substantial growth is observed in the vibrationless state on the time scale of vibrational relaxation. We estimate that only about 15% of the nascent CD3 population is formed in the vibrational states we detect: ν2=1 and 2, ν3=1, and the vibrationless state. Most of the nascent methyl population is evidently spread among many undetected vibrational states. These results complement previous measurements of acetone photofragments by infrared emission, multiphoton ionization, and laser-induced fluorescence. Our inferred global vibrational distribution is consistent with a two-step fragmentation. [ABSTRACT FROM AUTHOR]

Time-resolved diode laser absorption spectroscopy has been used to study vibrational-to-vibrational energy transfer from highly excited azulene and azulene-d8 to carbon dioxide. The energetic azulene molecules were prepared by internal conversion (S2→S0) after excitation into the S2 state by a pulsed laser source of 337 nm photons. Relative populations of CO2 vibrational modes were determined and compared with predictions of a statistical model. The total vibrational energy transferred to CO2 was determined to be about 25% of the initial azulene energy. Similar experiments were carried out using 248 nm photons to excite azulene in the region of the S4 state. [ABSTRACT FROM AUTHOR]

We report the observation and assignment of a number of vibration–rotation transitions within 1-←1+ band of H3O+. The data are analyzed together with previously published measurements to yield a consistent set of rotational parameters that accurately reflects the vibration–rotation spectra involving the four lowest levels associated with the ν2 vibration of the ion. The analysis enables a precise estimate of the inversion splitting and the associated inversion-rotation spectrum of H3O+ which occurs in the far infrared. [ABSTRACT FROM AUTHOR]

The GeH radical has been detected in its ground 2Π state in the gas phase reaction of fluorine atoms with GeH4 by laser magnetic resonance techniques. Rotational transitions within both 2Π1/2 and 2Π3/2 manifolds have been observed at far-infrared wavelengths and rotational transitions between the two fine structure components have been detected at infrared wavelengths (10 μm). Signals have been observed for all five naturally occurring isotopes of germanium. Nuclear hyperfine structure for 1H and 73Ge has also been observed. The data for the dominant isotope (74GeH) have been fitted to within experimental error by an effective Hamiltonian to give a set of molecular parameters for the X 2Π state which is very nearly complete. In addition, the dipole moment of GeH in its ground state has been estimated from the relative intensities of electric and magnetic dipole transitions in the 10 μm spectrum to be 1.24(±0.10) D. [ABSTRACT FROM AUTHOR]

A Doppler-limited high resolution vibronic spectrum of the methylene (CH2) b 1B1←a 1A1 transition in the near-infrared wavelength region has been obtained using transient absorption techniques. The radical was produced by 308 nm excimer laser photolysis of ketene (CH2CO) in a flow system. The analysis of this spectrum confirms the reassignments of some previously observed bands as well as the presence of new vibronic bands predicted by a recent ab initio calculation [Green et al., J. Chem. Phys. 94, 118 (1991)]. We also measured the Doppler broadened profiles of low-J rotational lines of CH2 under collisionless conditions. From the line profile analysis, we find that the 308 nm photolysis of ketene produces fragments with isotropic and uncorrelated velocity and angular momentum distributions. The Doppler profiles also provide a view of the coincident product state distributions. The measured Doppler profiles are consistent with a rotational distribution of CO produced in coincidence with low energy CH2 states given by statistical phase space theory. The vibrationally excited coincident CO appears, however, to be produced about 2–3 times more efficiently than is predicted by various statistical theories. A frequency modulation (FM) technique has been adopted to improve the sensitivity of the transient absorption experiment. There is a significant improvement in the observed signal to noise ratio of the CH2 spectrum over the dual beam method previously used. © 1994 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the use of time-resolved frequency modulation (FM) spectroscopy for the measurement of photochemically generated radicals. CN radicals from the 193 nm photodissociation of cyanogen (NCCN) have been detected using a phase-modulated cw Ti:sapphire ring laser, probing single rotational lines of the A 2Π←X 2Σ system. The combination of sensitivity with time and frequency resolution is more than adequate to record Doppler-broadened line shapes of collisionless photofragments. Significant signal-to-noise enhancement is demonstrated compared to a dual-beam transient absorption technique. [ABSTRACT FROM AUTHOR]