Lawrence S. Goldberg

Picosecond UV photolysis and laser-induced fluorescence probing of gas-phase nitromethane

Ground-state NO2 radicals are formed in <5 ps with a quantum yield of ≈ 70% in 264 nm photolysis of low pressure nitromethane, as revealed by laser-induced fluorescence probing. Raman shifted photolyzing pulses indicate the NO2 yield varies little with wavelength, although a small yield of excited state NO*2 produced at 264 nm increases significantly at 238 nm.

OPTICAL PARAMETRIC OSCILLATION IN LITHIUM IODATE

Optical parametric oscillation has been achieved in LiIO3 pumped by a pulsed ruby laser, with tunable output to 4μ. Operation of the singly resonant oscillator is described and threshold measurements are compared with theory.

Narrow-Bandwidth Tunable Infrared Difference-Frequency Generation at High Repetition Rates in LiIO 3

Difference-frequency generation was studied in LiIO(3) with a narrow-bandwidth tunable dye laser pumped at high pulse-repetition rates by a frequency-doubled Nd:YAG laser. Tunable infrared radiation of 0.1, cm(-1) bandwidth was generated from 1.25 mum to 1.60 mum and from 3.40 mum to 5.65 mum by mixing rhodamine B dye laser cavity radiation with the 1.064-mum or 532-nm output of the Nd:YAG laser.

Singly resonant CdSe parametric oscillator pumped by an HF laser

A transverse excitation HF laser has been used to pump a singly resonant CdSe infrared parametric oscillator. Tunable output has been generated over the range 4.3–4.5 μm and 8.1–8.3 μm with peak powers of up to 800 W. By a combination of angle tuning and selection of HF pump wavelength, CdSe is phase matchable for generation of radiation from 3.0–4.8 μm and 7.8–22 μm.

Time-resolved C2 swan emission from short-pulse UV fragmentation of CO: evidence for two C2 formation mechanisms

Abstract We have studied C 2 Swan (d 3 Π → a 3 Π u ) emission resulting from multiphoton UV excitation of CO. Population of d 3 Π proceeds through distinct early and late processes, the former giving rise only to normal Swan emission. The late process is responsible for v = 6 enhancement (high-pressure bands), and it dominates time-averaged emission in an bands for ⪖ 10 Torr of CO.

UV short‐pulse fragmentation of isotopically labeled acetylene: Studies of emission with subnanosecond resolution

We have studied fragment emission in the spectral range 200 to 900 nm, following photolysis of acetylene (1–10 Torr) by short ultraviolet pulses (25 ps, 266 nm, 2.5–12 mJ). The dominant component of emission is the C2 d 3Πg→a 3Πu Swan system. Weak singlet C2 emission is observed. CH is also observed, in the A 2Δ→X 2Π and C 2Σ+→X 2Π systems. With isotopic labeling and with streak camera recording, we have demonstrated distinct unimolecular and intermolecular processes yielding C2 d 3Πg. The unimolecular channel proceeds with risetime τ1 = 215±15 ps, while the pressure‐dependent intermolecular component develops over a period of nanoseconds. We discuss the underlying kinetics and the mechanistic implications of the 25 ps excitation.

OPTICAL SECOND‐ AND THIRD‐HARMONIC GENERATION IN CHOLESTERYL NONANOATE LIQUID CRYSTAL

Laser‐induced generation of second‐ and third‐harmonic radiation is studied in cholesteryl nonanoate and in other cholesteric and nematic liquid crystals. No second harmonic is observed in any of the liquid‐crystal mesomorphic phases, indicating their ordered molecular arrangements to be centrosymmetric. Large changes in third‐harmonic intensity occur at several phase transitions, and their possible origins are considered.

Short Pulse Absorption Spectroscopy of Nitromethane Photolysis

We are applying short pulse techniques to studies of small molecular fragments, stable or reactive radical species emergent as the primary products of unimolecular decomposition of simple photolabile materials. Nitro-methane (CH3NO2) photolysis has been the subject of extensive prior investigations under varying conditions of irradiation wavelength, phase, temperature, and with active additives. The identity of the primary products and the kinetics of the reaction remain unresolved, although several mechanisms have been advanced to explain the observed products. From nitromethane vapor the list of major final products which have been reported [1] includes CH3ONO, CH2O, CH3NO, NO, and N2O; minor final products include H2, CH4, N2, and CO; and a number of reaction intermediates have been reported, notably NO2 and HNO. As an inferred early product, NO2 was estimated to have a quantum yield as large as 0.6, and this fragment has been our principal object of study to date.