Search

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]

The study stated that the b(super ~) (super 1) B(sub 1) - a(super ~)(super 1)A(sub 1) spectrum of methylene between 9317 and 9610 cm(super -1) were recorded using frequency-modulated diode laser absorption spectroscopy. A comparison of the results of the recent high-level quantum chemical calculations suggested that the further refinement of the potential energy surface of the b(super ~) state near its equilibrium would be done.

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]

The origin of the cation ground electronic state was used to record the photoinduced Rydberg ionization spectrum of third excited electronic state of phenylacetylene cation. It was found that the vibrational structure of C state of phenylacetylene cation is well resolved, although the line widths are not quite as narrow as other substituted benzenes.

New spectra of methylene, CH2, in the near-infrared are obtained following 308 nm photolysis of ketene, CH2CO. Nascent photofragment Doppler spectra and thermalization kinetics vary systematically with the energy of the absorbing level, providing additional information to support or refute spectroscopic assignments made on the basis of the frequency measurements and combination differences.

The photodissociation experiments on CHBr3 at 248 nm were performed using VUV ionization photofragment translational spectroscopy. It is concluded that the most important photodissociation channel of CHBr3 in the UV region is C-Br bond breaking.

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.