Karen J. Castle

Direction of the transition dipole moment of nitrobenzene determined from oriented molecules in a uniform electric field

The direction of the transition dipole moment of nitrobenzene between 230 and 250 nm was determined by orienting gas-phase molecules in a strong, uniform electric field. Oriented nitrobenzene was photodissociated with linearly polarized light, and the NO fragments were detected by resonantly enhanced multiphoton ionization (REMPI). When the polarization direction of the photolysis laser was perpendicular (rather than parallel) to the orientation field, a 44% enhancement in the NO signal was observed. This implies a predominantly perpendicular relationship between the transition dipole and the permanent dipole. However, the experimentally observed enhancement falls below that expected of a pure perpendicular transition, indicating the presence of a second potential-energy surface that is simultaneously accessed through a parallel transition. Quantitative analysis indicates that the parallel transition contributes 20% of the overall oscillator strength.

Directions of transition dipole moments of t-butyl nitrite obtained via orientation with a strong, uniform electric field

A strong, uniform electric field was used to orient supersonically cooled t-butyl nitrite for measurements of directions of transition dipole moments. The oriented sample was dissociated with linearly polarized light, and the NO fragments were studied by (1+1) REMPI through the A 2Σ+ state. At photolysis wavelengths of 365.8 and 351.8 nm, there was a 47% enhancement in the NO signal when the photolysis beam was polarized perpendicular to the orientation field, implying a perpendicular relationship between the transition dipole of the S1 state and the permanent dipole. Photodissociation at 250 and 224 nm showed the opposite trend, with a 46% enhancement in the NO signal when the photolysis beam was polarized parallel to the orientation field. The transition dipole of the S2 state was therefore determined to be parallel to the permanent dipole. This experiment demonstrates the application of brute force orientation for obtaining directions of transition dipole moments.

Photodissociation of t-butyl nitrite between 220 and 250 nm: internal energy distribution of NO

Abstract Internal energy distribution of the NO product from photodissociation of t -butyl nitrite in the region between 220 and 250 nm are presented. The rotational distribution for all the vibrational levels are similar throughout the region, with a dominant Gaussian-like highly rotationally ( J ) excited component and a minor low- J component. Increase in the dissociation energy has a dramatic effect on the vibrational distribution: at 250 nm, the v ″=0 channel dominates, while at 220 nm, the v ″=0 channel is undetectable using our apparatus. These observations reveal new information on the S 2 surface and the dissociation dynamics of t -butyl nitrite.

Vibrational Relaxation of O3(ν2) by O(3P)

Laboratory measurements of the rate coefficient for quenching of O3(ν2) by ground-state atomic oxygen, kO(ν2), at room temperature are presented. kO(ν2) is currently not well known and is necessary for appropriate nonlocal thermodynamic equilibrium modeling of the upper mesosphere and lower thermosphere. In this work, a 266 nm laser pulse photolyzes a small amount of O3 in a slow-flowing gas mixture of O3, Xe, and Ar. This process simultaneously produces atomic oxygen and increases the temperature of the gas mixture slightly, thereby increasing the population in the O3(ν2) state. Transient diode laser absorption spectroscopy is used to monitor the populations of the O3(ν2) and ground vibrational states as the system re-equilibrates. Relaxation rates are measured over a range of quencher concentrations to extract the rate coefficient of interest. The value of kO(ν2) was determined to be (2.2 ± 0.5) × 10(-12) cm(3) s(-1).