We present a new technique for the detection of molecules desorbed from an ice surface using broad-band millimeter-wave rotational spectroscopy. The approach permits interrogation of molecules that have undergone the slow warmup process of temperature-programmed desorption (TPD), analogous to the warmup phase of icy grains in the interstellar medium as they approach the central protostar. The detection is conformer- and isomer-specific and quantitative, as afforded by chirped-pulse rotational spectroscopy.
View Article and Find Full Text PDFInfrared multiphoton excitation is combined with UV excitation and state-resolved probes of Cl(P), Cl*(P), and HCl to study the photochemistry of propargyl chloride. The results show evidence both of infrared multiphoton dissociation on the ground electronic state and infrared multiphoton excitation followed by UV dissociation. The results are interpreted with the aid of a full characterization of the stationary points on the ground state using ab initio methods, as well as our recent experimental and theoretical characterization of the UV photochemistry of the molecule.
View Article and Find Full Text PDFThe UV photodissociation of isoxazole (c-CHNO) is studied in this work by chirped-pulse Fourier transform mm-wave spectroscopy in a pulsed uniform Laval flow. This approach offers a number of advantages over traditional spectroscopic detection methods due to its broadband, sub-MHz resolution, and fast-acquisition capabilities. In coupling this technique with a quasi-uniform Laval flow, we are able to obtain product branching fractions in the 193 nm photodissociation of isoxazole.
View Article and Find Full Text PDFIsomer-specific detection and product branching fractions in the UV photodissociation of the propargyl radical is achieved through the use of chirped-pulse Fourier-transform mm-wave spectroscopy in a pulsed quasi-uniform flow (CPUF). Propargyl radicals are produced in the 193 nm photodissociation of 1,2-butadiene. Absorption of a second photon leads to H atom elimination giving three possible CH isomers: singlets cyclopropenylidene (c-CH) and propadienylidene (l-CH), and triplet propargylene (HCCCH).
View Article and Find Full Text PDFSinglet dihydroxycarbene (HOC̈OH) is produced via pyrolytic decomposition of oxalic acid, captured by helium nanodroplets, and probed with infrared laser Stark spectroscopy. Rovibrational bands in the OH stretch region are assigned to either trans,trans- or trans,cis-rotamers on the basis of symmetry type, nuclear spin statistical weights, and comparisons to electronic structure theory calculations. Stark spectroscopy provides the inertial components of the permanent electric dipole moments for these rotamers.
View Article and Find Full Text PDFWe present a DC slice imaging study of roaming dynamics in the photodissociation of the nitrate radical, NO3, contrasting pure visible excitation with a combination of visible and CO2 laser excitation at 10.6 μm. Images of specific rotational levels of NO are seen to reflect dissociation on the ground and first excited electronic states, as reported in previous work.
View Article and Find Full Text PDFWe have developed a new experimental method allowing direct detection of the velocity dependent spin-polarization of hydrogen atoms produced in photodissociation. The technique, which is a variation on the H atom Rydberg time-of-flight method, employs a double-resonance excitation scheme and experimental geometry that yields the two coherent orientation parameters as a function of recoil speed for scattering perpendicular to the laser propagation direction. The approach, apparatus, and optical layout we employ are described here in detail and demonstrated in application to HBr and DBr photolysis at 213 nm.
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