Patrick H. Vaccaro

Detection of stimulated emission pumping via degenerate four‐wave mixing

The stimulated emission pumping (SEP) process has been detected by means of a newly developed zero‐background scheme based upon the Doppler‐free technique of degenerate four‐wave mixing (DFWM) spectroscopy. Rotationally resolved SEP‐DFWM spectra recorded for the (02000) vibrational level of CS2 X 1∑+g exhibit signal‐to‐noise ratios approaching 1000:1.

The vibronically-resolved emission spectrum of disulfur monoxide (S2O): An algebraic calculation and quantitative interpretation of Franck–Condon transition intensities

Emission spectra obtained from jet-cooled disulfur monoxide (S2O) molecules have been interpreted by means of a novel Lie algebraic formalism that makes possible the facile evaluation of multidimensional Franck–Condon factors. Fluorescence accompanying selective excitation of isolated vibronic bands in the S2O C 1A′←X 1A′(π*←π) absorption system has been dispersed under moderate spectral resolution, allowing assignment of ground state levels possessing up to 20 quanta of vibration in the ν2 S–S stretching mode [Evib(X)⩽13 900 cm−1]. Aside from providing a rigorous and economical description for the inherently anharmonic nature of highly-excited polyatomic species, our algebraic approach enables quantitative information on molecular wavefunctions to be extracted directly from spectroscopic data. The emerging picture of S2O vibrational dynamics suggests that the X 1A′ potential surface is substantially more “local” in character than the C 1A′ manifold. While the observed pattern of X 1A′ vibrational e...

Conformational effects on optical rotation. 2-Substituted butanes.

The specific rotations of 2-substituted butanes (X = F, Cl, CN, and HCC) were calculated at the B3LYP/aug-cc-pVDZ level as a function of the C-C-C-C torsion angle. The results for the four compounds are remarkably similar, despite large differences in the electronic transition energies. The temperature dependence of the specific rotations for 2-methylbutyronitrile and for 2-chlorobutane was studied to give experimental information about the effect of the torsion angle on the specific rotation. The results were in good accord with B3LYP/aug-cc-pVDZ calculations. The specific rotations derived from the study of 2-chlorobutane are similar to those previously obtained for 3-chloro-1-butene, indicating that the double bond does not have a large effect on the optical rotations, but it did lead to a large difference between calculated and observed specific rotations.

Optical rotatory dispersion of 2,3-hexadiene and 2,3-pentadiene.

The specific rotation of (P)-2,3-hexadiene (1) was measured as a function of wavelength for the gas phase, the neat liquid, and solutions. There was a surprisingly large difference between the gas phase and condensed phase values. The specific rotation was calculated using B3LYP and CCSD, and the difference in energy between the three low energy conformers was estimated at the G3 level. The Boltzmann-averaged CCSD-calculated rotations using the gauge independent velocity gauge representation, as well as the B3LYP values, are in agreement with the gas-phase experimental values. In order to avoid possible problems associated with the conformers of 1, 2,3-pentadiene (2) also was examined. Here again, there was a large difference between the gas-phase and condensed-phase specific rotations, with the CCSD velocity gauge (and B3LYP) results being close to the gas-phase experimental values. The possibility that 2,3-pentadiene could be distorted on going from the gas to liquid phase, thereby accounting for the effect of phase on the specific rotation, was examined via a Monte Carlo statistical mechanics simulation. No effect on the geometry was found. Specific rotations of 1 found in solutions were similar to those for the liquid phase, indicating that the phase difference was not due to association.

Degenerate four-wave mixing spectroscopy as a probe of orientation and alignment in molecular systems

Degenerate four-wave mixing (DFWM) spectroscopy is shown to provide a facile means for probing angular momentum (or rotational) anisotropy in nonequilibrated ensembles of gaseous molecules, with judicious selection of experimental conditions permitting quantitative determination of population distributions and Zeeman coherences for magnetic sublevels of the target species. A theoretical description of the nonlinear response induced under such circumstances is obtained by incorporating a state multipole expansion of the zero-order density operator into a perturbative (weak-field) treatment for the DFWM interaction. Aside from allowing the effects of incident field polarizations and phase-matching geometries to be considered in detail, this compact spherical tensor formalism provides guidelines for the extraction of spatial information from rovibronically resolved spectral data. Furthermore, these analyses have identified unusual polarization schemes that lead to signal generation only in the presence of ro...

Large Solvation Effect in the Optical Rotatory Dispersion of Norbornenone

The anomalously large chiroptical response of (1R,4R)-norbornenone has been probed under complementary vapor-phase and solution-phase conditions to assess the putative roles of environmental perturbations. Measurements of the specific rotation for isolated (gas-phase) molecules could not be reproduced quantitatively by comprehensive quantum-chemical calculations based on density-functional or coupled-cluster levels of linear-response theory, which suggests that higher-order treatments may be needed to accurately predict such intrinsic behavior. A substantial, yet unexpected, dependence of the dispersive optical activity on the nature (phase) of the surrounding medium has been uncovered, with the venerable Lorentz local-field correction reproducing solvent-mediated trends in rotatory dispersion surprisingly well, whereas more modern polarizable continuum models for implicit solvation performed less satisfactorily.

Rotation–tunneling analysis of the origin band in the tropolone π*←π absorption system

The tunneling-split origin band of the tropolone A 1B2–X 1A1 (π*←π) absorption system was interrogated under ambient, bulk-gas conditions by exploiting high-resolution degenerate four-wave mixing techniques. The inherent complexity of this spectral region was alleviated by performing polarization-resolved measurements, with judicious selection of transverse characteristics for the incident and detected electromagnetic fields enabling rovibronic transitions to be discriminated according to their attendant changes in rotational angular momentum, ΔJ. Quantitative simulation of recorded data sets showed the vibrationless level of the electronically excited state to be bifurcated by Δ0A=19.846(25) cm−1, representing a factor of 20 increase in proton-transfer efficiency over the corresponding level of the ground electronic state. Spectroscopic parameters extracted for the 0+ and 0− manifolds of A 1B2 tropolone yield unexpectedly large values of the inertial defect, ΔI0+A=−0.802(86) amu A2 and ΔI0−A=−0.882(89) ...

Laser spectroscopy of crossed molecular beams: The dissociation energy of BaI from energy‐balance measurements

Through application of energy‐balance arguments to the crossed‐beam reaction Ba(1S0)+HI(X 1Σ+) →BaI(X 2Σ+) +H(2S1/2), a lower limit for the BaI bond dissociation energy is determined to be D00(BaI) ≳76.8±1.7 kcal/mol (3.33±0.07 eV). Based on the upper bound of D00(BaI) ≲78.5±0.5 kcal/mol, as determined from earlier predissociation studies [M. A. Johnson, J. Allison, and R. N. Zare, J. Chem. Phys. 85, 5723 (1986)], we recommend a BaI bond strength of 77.7±2.0 kcal/mol (3.37±0.09 eV). This dissociation energy is more than 5 kcal/mol higher than the previously accepted value of D00(BaI) as derived from mass spectrometric measurements.

High precision dipole moments in à 1A2 formaldehyde determined via Stark quantum beat spectroscopy

The high resolution technique of Stark quantum beat spectroscopy is used to examine the electric dipole moment function for the first excited singlet state (A 1A2) of formaldehyde‐h2 and formaldehyde‐d2. The high precision of these measurements (i.e., better than 5 parts in 104) enables detailed determination of a‐axis dipole moment components (μa ) for individual J=2 rovibronic levels in the ν4 out‐of‐plane bending mode. In the case of 21,1 rotational levels, we find μa (40)=1.4784(7) D and μa (41)=1.4678(4) D for H2CO. For D2CO the measured 21,1 dipole moments are μa (40)=1.4698(6) D, μa (41)=1.4693(3) D, and μa (43) =1.4786(7) D. The state‐specific variations in μa revealed by this study reflect the structural influences exerted by the pervasive S1∼S0 nonadiabatic interactions and the pyramidally distorted equilibrium configuration which characterize the A state of formaldehyde. The origin and experimental manifestation of the out‐of‐plane dipole moment component (μc ) in nonrigid A 1A2 formaldehyde...

A novel algebraic scheme for describing nonrigid molecules

Abstract An algebraic scheme for describing nonrigid polyatomic molecules is introduced and used to characterize bending motion in ‘floppy’ triatomic/tetratomic species. The salient features of quasi-linear and quasi-bent molecules are classified systematically. In particular, the ( ν 5 ) bending vibration supported by the ground electronic state of fulminic acid (HCNO/DCNO) is shown to exhibit the predicted behavior for nonrigidity. Likewise, the ( ν 2 ) bending motion of magnesium hydroxide (MgOH/MgOD) demonstrates quantitative application of this novel approach to problems of spectroscopic interest. Effective potential energy functions for these large-amplitude degrees of freedom are extracted by exploiting the method of coherent states.