Browsing by Author "Reiche, F."

Using the technique of double-resonance overtone photofragment spectroscopy (DROPS), we have measured rotationally resolved vibrational overtone transitions to the previously unobserved 5v(1), 6v(1), and 7v(1) levels of gas-phase trans-nitrous acid (HONO) in its electronic ground state. Observing the onset of dissociation from different rovibrational states of 5v(1) near threshold determines the HO-NO bond energy to be D-0=16 772 +/- 14 cm(-1). Observed spectral splittings and broadening of individual rovibrational transitions provide quantitative data on the rate and extent of collision free vibrational energy redistribution that would result after coherent ultrashort pulse excitation. In parallel with these frequency domain measurements, we determine the unimolecular dissociation rates directly in time for trans-HONO molecules excited to several rotational states near threshold. The combination of time- and frequency-resolved data allows us to estimate the linewidth contributions from the finite dissociation lifetime of the molecule. Our results reveal intramolecular dynamics that are clearly not a simple function of the vibrational energy but rather depend sensitively upon specific couplings and, in turn, on the vibrational character of the individual states excited. (C) 2000 American Institute of Physics. [S0021-9606(00)00320-2].

Linewidths, unimolecular dissociation rates and product state distributions (PSDs) have been measured for single rovibratational states of the nu(1)=5-7 levels of gas-phase trans-nitrous acid (HONO) by double-resonance overtone photofragment spectroscopy (DROPS). The linewidth measurements, together with the unimolecular dissociation rates in 5nu(1), suggest that the intramolecular dynamics are not statistical but rather depend sensitively upon specific intramolecular couplings and the vibrational character of the initial state. Comparison with calculated rate constants from statistical unimolecular rate theory reveals that intramolecular vibrational energy redistribution (IVR) is the rate determining step in the dissociation of HONO subsequent to vibrational overtone excitation. Despite this, we find the measured product state distributions to be close to the predictions of statistical theory. We explain these observations in terms of a simple tier model incorporating hierarchical IVR. The experimental findings underscore the importance of the preparation technique, and hence the nature of the initially excited state, in determining the subsequent intramolecular dynamics. (C) 2002 American Institute of Physics.

Loosely bound states of jet cooled NO2 just below the first dissociation threshold D-0, with binding energies E-b between 0.8 and 59.3 cm(-1), have been investigated using pulsed VIS/UV optical double resonance spectroscopy. The measured UV spectra of these states in a spectral region where free NO absorbs have been found to depend strongly on the binding energy E-b = D-0 - E. This suggests that the states just below the dissociation threshold D-0 may be regarded (at least in part) to belong to a family of states corresponding to a large amplitude motion of an "oxygen atom" and a "NO fragment", Such states, typical for loosely bound nonrigid molecules or van der Waals complexes, are unusual for chemically bound molecules, in this paper we are describing first experiments in which we obtained direct evidence for their existence in NO2. Most of the absorptions from the loosely bound states terminate on a dissociative potential energy surface (PES), so that the corresponding spectrum is a broad unstructured feature, with a blue shift (compared to free NO) increasing with binding energy. Very weak bound-bound transitions have also been observed. The analogy to spectra of NO/Ar van der Waals complexes is discussed.