T. Rick Fletcher

Atmospheric reactions of vibrationally excited greenhouse gases : SH+N2O(n, 0, 0)

Abstract Rate coefficients for the reactions of SH + N 2 O(ν 1 , 0, 0) → HSO (A 2 A′) for ν 1 = 0 and 1, and HSO (A 2 A′) + N 2 O → products, are measured at room temperature. SH is formed by pulsed laser photolysis of H 2 S and vibrationally excited N 2 O is created using the stimulated Raman technique. The reaction kinetics are followed by chemiluminescence of HSO. The results show that the reaction between SH and unexcited N 2 O has a rate coefficient equal to 1.3 (±0.14) × 10 −11 cm 3 molecule −1 s −1 , while N 2 O (1, 0, 0) reacts with a rate coefficient at least 2.2. times larger.

Efficient ablation of an organic polymer by a laser driven shock wave

A new method for the efficient removal of an organic polymer is described. Fragments on the order of 1 mm diam are produced when a 0.02 J, 5 ns pulse of 532 nm light is focused into a red organic polymer that is coated on the front side of a transparent window. Quantities as large as 200 μg and craters as deep as 200 μm have been ablated in a single laser pulse. The large mass particles resulting from the ablation generate high transient pressures on the order of 100 kbar.

A simple method for quantitative comparisons of mode specific chemistry using stimulated Raman excitation

Stimulated Raman excitation (SRE) of polyatomic molecules provides an efficient and general means for studies of mode specific chemistry. Here we present a simple method for quantifying the amount of vibrationally excited polyatomic molecules that can be created using SRE and demonstrate that excitation of vibrational overtones in polyatomics is also possible.

The photofragmentation dynamics of 3-cyclopentenone in the gas phase

Abstract Vibrational energy disposal to the CO product of the photofragmentation of 3-cyclopentenone has been studied by time-resolved CO laser absorption spectroscopy. The nascent vibrational distribution of CO is N 0 = 7.88, N 1 = 0.187, N 2 = 0.024. The measured vibrational distribution is substantially colder than that predicted on statistical grounds. This indicates that the product fragments become vibrationally decoupled from one another before the full reaction exoergicity has been released. Our results suggest that such decoupling may occur at the dissociation transition state.

TECHNIQUES FOR STIMULATED RAMAN EXCITATION AND CARS DETECTION OF RADICALS CREATED BY PHOTODISSOCIATION

Abstract CARS and SRE of radicals created by photodissociation were attempted using a laser system with a 1 cm −1 bandwidth. The difficulty of observing these spectra due to interference from non-resonant background and the resulting asymmetric CARS lineshape is discussed and demonstrated with N 2 O, HS and OH. Polarization filtering techniques are described and improvements resulting from their use are presented. The HS radical, which has a Raman frequency on the low energy side of the H 2 S precursor molecule, could not be detected, while OH, which has a Raman frequency on the high energy side of the HNO 3 precursor molecule, was easily observed.

Vaporization of Nonvolatile and Matrix-Isolated Molecules Using a Novel Laser Vaporization Technique

A technique is described which uses a laser-generated high-velocity polymer flyer to vaporize nonvolatile molecules and low-temperature matrix-isolated species. Morphology studies, time-of-flight measurements, static collection methods, and real-time laser-induced fluorescence were used to characterize the new vaporization technique. Cr(CO)6 in a mineral oil glass at 77 K, rhodamine dye, and naphthalene were vaporized in these experiments.

Using Λ-doublet ratios to understand collision geometry in direct bimolecular reactions

A model is developed which can be used to understand how collision geometry can dictate product state lambda doublet ratios in direct bimolecular reactions that produce diatomic products in a Π molecular energy state. The predicted lambda doublet ratio is A′/A″=2 for unconstrained collision geometry, while the two limiting case geometries of exclusive end-on collisions and exclusive side-on collisions yield lambda doublet ratios of A′/A″=∞ and A′/A″=1, respectively. The theory is derived and used to interpret the experimental lambda doublet ratio of OH (v=0) from the H+N2O→OH+N2 reaction.

Time‐resolved FTIR emission studies of molecular photofragmentation initiated by a high repetition rate excimer laser

The availability of high repetition rate (≳300 Hz) excimer lasers provides new opportunities for studies of molecular processes by time‐revolved FTIR spectroscopy. An overview of the technique is given and state resolved infrared emission results are presented for the triatomic radical, C2H, generated by photolysis of C2H2 at 193 nm. Electronic emission from the low lying A2Π state of C2H is observed, along with high vibrational levels of the ground state which gain intensity by coupling with the vibrationless level of A2Π.