Zane Arp

Electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy of nitric oxide

A dual-pump, electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy (CARS) technique for the measurement of minor species concentrations has been demonstrated. The frequency difference between a visible Raman pump beam and Stokes beam is tuned to a vibrational Q-branch Raman resonance of nitric oxide (NO) to create a Raman polarization in the medium. The second pump beam is tuned into resonance with rotational transitions in the (1,0) band of the A2Σ+–X2Π electronic transition at 236 nm, and the CARS signal is thus resonant with transitions in the (0,0) band. We observe significant resonant enhancement of the NO CARS signal and have obtained good agreement between calculated and experimental spectra.A dual-pump, electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy (CARS) technique for the measurement of minor species concentrations has been demonstrated. The frequency difference between a visible Raman pump beam and Stokes beam is tuned to a vibrational Q-branch Raman resonance of nitric oxide (NO) to create a Raman polarization in the medium. The second pump beam is tuned into resonance with rotational transitions in the (1,0) band of the A2Σ+–X2Π electronic transition at 236 nm, and the CARS signal is thus resonant with transitions in the (0,0) band. We observe significant resonant enhancement of the NO CARS signal and have obtained good agreement between calculated and experimental spectra.

Spectroscopic determination of the two-dimensional vibrational potential energy surfaces for the ring-puckering and ring-flapping modes of indan in its S0 and S1(π,π*) electronic states

The vapor-phase far-infrared, mid-infrared, ultraviolet, Raman, and laser-induced fluorescence spectra of indan have been recorded and analyzed. The far-infrared spectra, which are very similar to those previously reported, together with the Raman and dispersed fluorescence (SVLF) spectra of the jet-cooled molecules were used to reassign the ring-puckering and ring-flapping energy levels for the S0 ground state. These were then utilized to calculate a two-dimensional vibrational potential energy surface (PES) which nicely fits all of the assigned puckering and flapping levels. The PES has a barrier of 488 cm−1 as compared to a previously reported value of 1979 cm−1, which was based on a one-dimensional analysis and earlier assignments. The dihedral angle of puckering is ±30°. Fluorescence excitation spectra of jet-cooled indan together with ultraviolet absorption spectra were used to assign the flapping and puckering levels in the S1(π,π*) electronic excited state. The PES for this state has a barrier of ...

Raman spectroscopy of vapors at elevated temperatures

The most effective way to obtain high quality vapor-phase Raman spectra is to heat the samples to increase their vapor pressure. Many samples can be heated to 350 °C and higher without decomposition. We have designed a simple Raman cell to allow these high temperature studies to be carried out. The high-temperature Raman spectra of nine molecules will be presented and discussed. Most of these are non-rigid molecules containing aromatic rings for which vibrational potential energy surfaces have been determined from their spectra. Two molecules (p-cresol and 3-methylindole) are model compounds for amino acids and their vapor-phase spectra are characteristic of environments with no hydrogen bonding.

Laser-induced fluorescence, electronic absorption, infrared and Raman spectra, and ab initio calculations of 1,2-dihydronaphthalene: Investigation of the out-of-plane ring modes for the ground and S1(π,π*) excited states

The laser-induced fluorescence spectra and dispersed fluorescence spectra of jet-cooled 1,2-dihydronaphthalene have been analyzed to investigate the ring inversion process in both the S0 and S1(π,π*) excited states. Ultraviolet absorption, infrared, and Raman spectra were also recorded to complement the analyses. Ab initio calculations predict the inversion process to involve four out-of-plane ring motions, and linear combinations of these were made to model the inversion process. The data show the barrier to inversion in the ground state to be 1363±100 cm−1 (the triple-zeta ab initio value is 1524 cm−1). The experimental data indicate that the barrier increases substantially in the excited state, for which the calculated barrier is 1526 cm−1 with a CIS/6-311+G(d) basis set.

Fluorescence and ultraviolet absorption spectra and structure of coumaran and its ring-puckering potential energy function in the S1(π,π*) excited state

The fluorescence excitation (jet cooled), single vibrational level fluorescence, and the ultraviolet absorption spectra of coumaran associated with its S1(pi,pi*) electronic excited state have been recorded and analyzed. The assignment of more than 70 transitions has allowed a detailed energy map of both the S0 and S1 states of the ring-puckering (nu45) vibration to be determined in the excited states of nine other vibrations, including the ring-flapping (nu43) and ring-twisting (nu44) vibrations. Despite some interaction with nu43 and nu44, a one-dimensional potential energy function for the ring puckering very nicely predicts the experimentally determined energy level spacings. In the S1(pi,pi*) state coumaran is quasiplanar with a barrier to planarity of 34 cm(-1) and with energy minima at puckering angles of +/-14 degrees. The corresponding ground state (S0) values are 154 cm(-1) and +/-25 degrees . As is the case with the related molecules indan, phthalan, and 1,3-benzodioxole, the angle strain in the five-membered ring increases upon the pi-->pi* transition within the benzene ring and this increases the rigidity of the attached ring. Theoretical calculations predict the expected increases of the carbon-carbon bond lengths of the benzene ring in S1, and they predict a barrier of 21 cm(-1) for this state. The bond length increases at the bridgehead carbon-carbon bond upon electron excitation to the S1(pi,pi*) state give rise to angle changes which result in greater angle strain and a nearly planar molecule.

Infrared and ab initio study of the relative stability of the 3-fluoropropene-hydrogen chloride Van der Waals complex

The mid-infrared spectra (4000−400 cm-1) of complexes between the cis and gauche conformers of 3-fluoropropene, CH2CHCH2F, and HCl have been recorded in liquefied argon at temperatures ranging from...