J. P. Judish

Kinetic studies of N2 and N2–SF6 following proton excitation

Low intensity proton pulses have been used to excite pure N2 and N2–SF6 mixtures. By accumulating time and wavelength resolved information on the fluorescence that arises from a very large number of pulses, we have obtained data that are free from effects due to superelastic collisions with electrons or other nonlinear effects such as the interactions between ions or excited neutral species. For N2 the natural lifetimes of N2(C 3Πu) and N2+(B 2Σu+) were determined by monitoring N2(C 3Πu→B 3Πg) and N2+ (B 2Σu+→X 2Σg+) states, respectively. The quenching rates of N2(C 3Πu) and N2+(B 2Σu+) in different vibrational states by pure N2 and by SF6 were obtained. The addition of small concentrations of SF6 altered the late time behavior of the N2(C 3Πu→B 3Πg) transitions, and the addition of much higher concentrations of SF6 caused a large increase in the intensity of the 3371 A (C→B) transition.

Kinetic studies of ArF* and Ar2F* in proton‐excited Ar–F2 mixtures

A detailed neutral kinetic scheme for the production and quenching of ArF* and Ar2F* is provided from time‐resolved and time‐integrated spectra for proton‐excited Ar–F2 mixtures at various partial pressures. The mechanism for the production of Ar2F* is concluded mainly from the three‐body quenching of ArF* by argon. It is also found that the efficiency to produce ArF* by Ar2* (1u) with F2 is comparable to Ar(3P2) colliding with F2. The two‐body quenching rate constants of ArF* and Ar2F* by F2, the three‐body quenching rate constant of ArF* by Ar, and the radiative lifetime of Ar2F* are also obtained. The mechanism of ion–ion recombination to produce ArF* is briefly discussed.

Charge transfer and Penning ionization of N2, CO, CO2, and H2S in proton excited helium mixtures

In this work we report on studies of the kinetics of He–N2 mixtures excited by a proton beam. The rate of charge transfer from both atomic and molecular helium ions to N2 was studied using a time‐resolved technique to measure the fluorescence of N+2(B 2Σ+u→X 2Σg). The population of N+2(B2Σ+u) through a Penning ionization process from metastable helium atoms or molecules is observed, and the Penning ionization and charge transfer process which populates this state in a He–N2 laser is discussed. The charge transfer rate constants of He+–CO, He+–CO2, He+–H2S, and He+2–CO are also measured.

Kinetic studies of Ar–N2–SF6 mixtures following proton excitation

Time resolved studies h!ve b%en made of both the vacuum ultraviolet (vuv) and the uv‐visible radiation due to pulsed proton excitation of Ar–N2, Ar–SF6, and Ar–N2–SF6 mixtures. Photon emissions from Ar and from N2 (due to energy transfer from Ar) were both studied, using time resolved spectroscopic techniques. In the Ar–N2 mixtures the decay of N2(C 3Πu→B 3Πg) is much slower than the natural lifetime of N2(C 3Πu). In the case of the (0–1) band for the transition N2(C 3Πu→B 3Πg), the decay follows Ar(3P2); while the decay of the (3–1) band follows that of Ar(1P1). Decay rates of (1–0) and (2–1) are between the decay of Ar(1P1) and Ar(3P2). From least squares fitting, it was found that 68% of v′=2 of N2(C 3Πu) comes from Ar(1P1) and 32% from Ar(3P2); while 86% of v′=1 comes from Ar(3P2) and 14% from Ar(1P1). Since the energy precursor of N2(C 3Πu)v′=3 is Ar(1P1), it is clear that the Wigner spin conservation rule does not hold rigorously in this energy transfer process. Quenching rates of Ar(1P1), Ar(3P2), ...

Energy-transfer processes in proton-excited Ar-Xe and Ar-F2 mixtures

Mixtures of Ar and Xe and of Ar and F2 were excited with pulsed proton beams and time-resolved spectroscopy was employed to study energy transfer from Ar to Xe or F2. Measurements were made of the quenching rates of Ar(1P1), Ar(3P1) and Ar2*(1u) by Xe and F2. The energy precursors of Xe(1P1), Xe(3P1) and Xe(2P32/(5d), J=1) are identified from Ar2*(1u), Ar2*(1u) and Ar(3P1), respectively. Excimers of ArF are populated by Ar(3P2) and Ar2*(1u) from the energy-transfer process and the ion-ion recombination process by argon and fluorine ions. The kinetics of Ar-F2, Ar-Kr-F2 and Ar-Xe-F2 lasers are discussed.

Measurement of the diffusion coefficient of Li in argon

The diffusion coefficient of Li in Ar was measured at T=563 K. Free Li atoms were produced in a line source by laser photodissociation of LiI in an isothermal cell. The time change in the Li concentration was monitored by resonant laser ionisation of free Li atoms contained in a laser beam volume concentric with the initial line source. The product of the diffusion coefficient D and pressure p has the value pD=12.3+or-1.9 N s-1 (923+or-143 Torr cm2s-1). This is in very good agreement (5%) with a value predicted by a semi-empirical calculation.

Kinetics studies of Ne–N2 by proton excitation

Charge transfer processes in Ne–N2 mixtures excited by weak proton beams are studied through the time‐resolved fluorescence spectra of N+2(B2Σ+u→X2 Σ+g). Our experimental data give indirect evidence of the existence of Ne+3. We also report the first measurements of overall quenching rates for atomic and molecular neon ions at room temperature. The fluorescence spectra of Ne–N2 and the ratio of the fluorescence efficiency to that of He–N2 are also obtained.

Experimental studies of self-suppression of vacuum ultraviolet generation in Xe

Vacuum ultraviolet light in the range 116 nm to 117 nm was produced by using a two‐photon resonant four‐wave mixing scheme in Xe. The buildup of coherent cancellation of the two‐photon resonant transition employed in the generation of the vacuum ultraviolet, with resulting limitations imposed on the achievable vacuum ultraviolet intensity was investigated. Under certain predicted conditions, increases in the intensity of one of the pumping beams, ≊1500 nn infrared, or tuning this beam towards resonance with the 5p 57s(3/2)1 level of Xe led, not to increases, but decreases in the vacuum ultraviolet generated.

Effect of the coherent cancellation of the two‐photon resonance on the generation of vacuum ultraviolet light by two‐photon resonantly enhanced four‐wave mixing

Many of the most impressive demonstrations of the efficient generation of vacuum ultraviolet (VUV) light have made use of two‐photon resonantly enhanced four‐wave mixing to generate light at ωVUV =2ωL1±ωL2. The two‐photon resonance state is coupled to the ground state both by two photons from the first laser, or by a photon from the second laser and one from the generated VUV beam. We show here that these two coherent pathways destructively interfere once the second laser is made sufficiently intense, thereby leading to an important limiting effect on the achievable conversion efficiency.

New coherent cancellation effect involving four‐photon excitation and the related ionization

We describe here an effect which occurs when a first laser is tuned near a dipole allowed three‐photon resonance and a second laser is used to complete a dipole allowed four‐photon resonance between the ground state ‖ 0≳ and an excited state ‖ 2≳. In this process three photons are absorbed from the first laser and one photon from the second; so that if the ‖ 0≳ to ‖ 2≳ transition is two‐photon allowed the transition is also pumped resonantly by the third harmonic field due to the first laser and the second laser field. When the second laser is strong enough to cause strong absorption of the third harmonic light, and the phase mismatch, Δk is large and dominated by the nearby resonance, a destructive interference occurs between the pumping of the ‖ 0≳ to ‖ 2≳ transition by two‐ and four‐photon processes.