Double-resonance overtone photofragment spectroscopy of trans-HONO. I. Spectroscopy and intramolecular dynamics

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].

Electric dipole moments of highly excited molecular vibrational states

Patrice Theulé

In this work, new spectroscopic techniques have been developed to measure electric dipole moments of highly excited rovibrational states of small polyatomic molecules in the gas phase. These techniques make use of lasers arid of microwave synthesizers. They enable one to measure the change on a molecular system caused by applying an external electric field, which is called Stark effect and from this, extract the dipole moment. The first technique, called microwave Stark spectroscopy, makes use of microwave-optical double resonance combined to either laser induced fluorescence or vibrational predissociation detection. The second approach, called Stark induced quantum beat spectroscopy, relies on the time evolution of a coherently prepared molecular wavepacket in an electric field, using either electronic photodissociation or laser induced fluorescence for detection. These techniques have been applied to H2C0, HOCl, HDO, and H20 for whom the electric dipole moment have been measured for several highly excited rovibrational states of the ground electronic state. Using these experimental measurements, the dependence of the dipole moment vector, both in orientation and in magnitude, on the vibrational excitation is discussed. Moreover the experimental data are used to test ab initio calculations potential energy and dipole moment surfaces and to establish critical benchmarks for future improvements. Due to the rarity of dipole moment data for highly excited vibrational states and their central role in transition intensities, intermolecular forces and collisions, these measurements are of special importance for chemical and energy transfer processes in atmospheric sciences, combustion studies, planetology, or more generally in the whole quantitative spectroscopy field, where transitions intensities are at least as important as line frequency positions.

EPFL2003

Intramolecular energy transfer in highly vibrationally excited methanol. II. Multiple time scales of energy redistribution

Thomas Rizzo

State-selected spectra of the OH stretch overtones of methanol in the range of upsilon(1) = 3-8 reveal spectral splittings and broadenings that result from vibrational couplings within the molecule. We employ a two-color excitation technique in which an infrared pulse promotes jet-cooled methanol molecules to a single rotational state in upsilon(1) = 1 or 2 and a second visible or near-infrared laser pulse is scanned to record a vibrational overtone spectrum. The final vibrationally excited species are detected by infrared laser assisted photofragment spectroscopy. The implications of the spectra for vibrational dynamics in the time domain can be understood in terms of a hypothetical coherent excitation of relevant portions of the spectrum. The observed splittings and widths correspond to three time scales. The largest splittings imply subpicosecond oscillation of energy between the OH stretch and a combination with the C-H stretch (5 nu(1) double left right arrow 4 nu(1) + nu(2) and 6 nu(1) double left right arrow 5 nu(1) + nu(2)) or a combination with the COH bend (7 nu(1) double left right arrow 6 nu(1) + 2 nu(6)). Secondary time scales correspond to finer splittings and are thought to arise from low-order resonances with other vibrational states. We argue that the nonmonotonic energy dependence of the presence and extent of such secondary structure throughout the recorded spectra reflects the requirement of resonance with important zeroth-order states. The third time scale, represented by the widths of the narrowest features at each overtone level, reflects the onset of vibrational energy randomization. These widths increase exponentially with vibrational energy in the range 2 nu(1) up to 8 nu(1). At the highest energy (25 000 cm(-1)) the three time scales begin to converge, implying an irreversible decay of the OH stretch overtone in 300 fs. (C) 1999 American Institute of Physics. [S0021-9606(99)02317-X].

1999

Orientation, rotation and solvation of ions in helium nanodroplets

Verónica Oliver Álvarez de Lara

Helium nanodroplets are liquid, finite size, nanoscale helium clusters that have been employed since the 1990s as a matrix for high-resolution spectroscopy of molecules. Spectroscopic studies of ionic species inside helium droplets were not however realised until 2010, even though the potential information about their structure is of special interest since they are fundamental reaction intermediates in biochemistry. Additionally, the interaction of these species with the helium environment can be determined from these experiments, which is interesting from a fundamental point of view. To determine structural information and study product selectivity, we proposed using high electric fields to orient the molecular ions in helium nanodroplets. Pendular state spectroscopy of neutral polar molecules in helium nanodroplets in the presence of electric fields has previously given satisfactory results to achieve molecular orientation, allowing one to distinguish biomolecular isomers. Our first experiments validated the technique for neutral aniline since the polarisation difference ratios, which express the degree of orientation, obtained from the NH2-stretch transitions were well-reproduced by calculations. On the other hand, the measured degree of orientation of cationic aniline is noticeably smaller than calculations show. Even so, the variation of the degree of orientation with electric field lets us conclude that orientation of the ions in helium droplets was observed. The results of several experiments performed to investigate the possible effects influencing the apparent orientation of the cations did not lead to any straightforward explanation. Our study therefore shows that orientation of molecular ions indeed seems possible and leads the way to possible future experiments. To examine the formation of ionic-alkali solvation complexes in helium droplets upon photoionisation of the alkali, sodium, and understand the helium-dopant interaction we have performed ion spectroscopy. We found that several scenarios are possible after ionisation including dissociation, solvation and complete solvent evaporation depending on the ionisation process, droplet size and number of water ligands considered. Direct ionisation of sodium leads to solvation in the droplet, collision with the water molecules and formation of the ionic complexes if the droplet size is sufficiently large. For these conditions, rovibrational spectroscopy of Na(OH2)+ was performed to obtain the A rotational constant in helium droplets and thus determine if the dopant-helium interaction affects the moment of inertia. From the results, we could confirm the latter but could not deduce the extent to which this was the case. This study shows how the spectral information of the solvated ions is useful to understand complexation dynamics in helium droplets and acquire further understanding about the dopant-helium interaction.