Mark Sulkes

Collision induced relaxation of an electronically excited molecule: Evidence for low energy resonance enhanced vibrational deactivation

The paper reports a study of collision induced vibrational relaxation of electronically excited iodine. A novel technique is used to probe the energy dependence of the collision cross section, namely, variation in the position of illumination along the axis of a supersonic free jet. Experimental results are reported which indicate that the relaxation induced by collisions with extremely small relative kinetic energy is very efficient. The evidence available is consistent with a model in which the vibrational deactivation is enhanced by orbiting resonances or rotational resonances of the colliding pair.

Vibrational redistribution in jet-cooled perylene intermediate coupling case

Fluorescence excitation spectra (FES) and fluorescence spectra (FS) under single vibronic level excitation of perylene in a supersonic jet are reported. The fluorescence from the lowest vibronic levels of the S1 state is purely resonant while for δEvib > 62; 1600 cm−1 broad-band emission spectra weakly dependent on νexe are explained by vibrational redistribution with a characteristic time 5 <Tred < 100 ps. In the intermediate δEvib range (700–1600 cm−1) complex emission spectra correspond to a strong coupling between the doorway state and a small number of “dark” states. The data are discussed in terms of a model assuming a rapid variation of the anharmonic coupling constant with the overall change of the vibrational quantum number between interacting levels.

Spectroscopy of tryptophan derivatives in supersonic expansions: Addition of solvent molecules

The addition of solvent molecules, primarily water and some alcohols, to isolated tryptophan derivative molecules has been carried out using supersonic free jet techniques. The derivatives studied include 3‐indole acetic acid, 3‐indole propionic acid, N‐acetyltryptophan ethyl ester, tryptamine, and tryptophol. The primary spectroscopic method employed was laser induced fluorescence, with some individual bands also characterized by fluorescence lifetime measurements or dispersed emission spectra. Unlike solvent addition to indoles already studied, these tryptophan derivatives also offer polar addition sites on the 3‐indole chain. Addition to these sites, further removed from the electronic transition, produces solvent shifts of the complex origins that are small compared to the red shifts observed in indoles; small blueshifts occur in some instances. Competition between binding at the 3‐indole polar group and the indole moiety nitrogen sites is strongly unequal, with the polar 3‐indole chain sites favored,...

ANOMALOUSLY SHORT FLUORESCENCE LIFETIMES IN JET COOLED 4-HYDROXYINDOLE. EVIDENCE FOR EXCITED STATE TAUTOMERISM AND PROTON TRANSFER IN CLUSTERS

Abstract Jet cooled fluorescence lifetimes following excitation of S 1 origins have been measured for 4-hydroxyindole. The subnanosecond values obtained are anomalous compared to other substituted indoles, in particular 5-hydroxyindole. The short lifetimes are most likely related to excited state tautomerism in the bare 4-hydroxyindole. It was further found that one solvent molecule addition of triethylamine to 4-hydroxyindole is sufficient to induce excited state proton transfer, to a significantly greater extent than with 5-hydroxyindole.

Mode‐to‐mode energy transfer in 1B2 aniline induced by very low energy collisions with He

Collision induced vibrational relaxation of selected single vibronic levels of 1B2 aniline has been studied for the collision energy range ∼1–∼100 cm−1. We find that very low energy collisions with He lead to efficient and selective mode‐to‐mode energy transfer, with the collision energy required for efficient relaxation varying with the nature of the initial vibrational excitation. This dependence on initial vibrational state is consistent with the orbiting resonance model of the collision previously proposed.

Theoretical and experimental characterization of supersonic expansions from slit sources

Abstract Theoretical and experimental studies of supersonic expansions from slit sources are reported.

Conformational analysis of jet cooled tryptophan analogs by molecular mechanics: Comparison with experiment

The possibility of predicting the geometries of jet cooled conformers of tryptophan analogs via molecular mechanics, employing MM2 (1987), was studied by carrying out computations on six different analog molecules. For three of the analogs, tryptamine in particular, the computationally predicted gas phase conformer geometries could be compared with experimentally determined geometries obtained from jet cooled samples. Some conformer geometries agreed qualitatively with experiment but the match did not extend to all cases. Most notably, the MM2 calculations failed to predict the eclipsed conformers of tryptamine that were deduced experimentally via analysis of rotational constants. They similarly failed to predict a planar geometry conformation found for 3‐indole acetic acid. These two disagreements prompted a reconsideration of the possible geometries that yield the best match to the experimentally determined rotational constants for various jet cooled conformers. For tryptamine conformers D/E it was foun...

Very low energy collision induced vibrational relaxation of 1Au glyoxal

We report the results of a study of very low energy collision induced vibrational relaxation of 1Au glyoxal. The effective collision energy is found to be different for different initial vibronic states, and the final states populated are drawn from a small subset of all available final states. The observed pathways of relaxation are interpreted using a previously proposed orbiting resonance model for the He: glyoxal collisions. It is inferred that (i) single collision–single quantum transfers dominate the relaxation process, and (ii) when there are near degeneracies in the vibrational manifold the lowering of symmetry in the orbiting resonance can permit mode mixing that is prohibited in the isolated molecule.

Further evidence for enhanced vibrational relaxation induced by very-low-energy atom-polyatom collisions

Very-low-energy (<20 cm−1) Collisions of He With 1B2 toluene and 1B2 fluorobenzene have very large, mode-dependent, vibrational relaxation cross sections, whereas similar collisions of He with 1B2u benzene have vanishingly small cross sections. These data are consistent with a scattering resonance model for very-low-energy collision-induced vibrational relaxation.

10 μs FWHM gas pulse generation using a pulsed solenoid valve

A General Valve Series 9 pulsed solenoid valve, when equipped with a 3.3 Ω coil and driven by 400 μs low voltage (∼22–65 V) pulses, can stably produce gas pulses as little as 10 μs FWHM. This capability can enhance the performance of a skimmed supersonic molecular beam system.