Francis G. Celii

Detection of ground‐state atomic hydrogen in a dc plasma using third‐harmonic generation

Third‐harmonic generation (THG) was used to monitor ground‐state atomic hydrogen H(1s 2S1/2) in a dc plasma system. A 364.6 nm laser beam focused through H2 or CH4/H2 plasmas induced THG at 121.5 nm, near the atomic hydrogen 2p 2PoJ→1s 2S1/2 Lyman‐α transition. Both the intensity and frequency shift of the excitation spectra exhibited dependence on the plasma power. Absolute H atom concentration was estimated by comparing the frequency shift to that obtained in a calibrated microwave discharge flow system. The sensitivity was ∼4×1013 cm−3 (100 ppm). The measured atomic hydrogen densities were substantially less than in other diamond chemical vapor deposition methods and may explain the lower diamond deposition rates obtained with dc plasma systems of this type.

The [Btilde] 2π3/2←[Xtilde] 2π3/2 laser excitation spectrum of bromocyanogen cation: A rotational analysis

The laser excitation spectrum of the [Btilde] 2π3/2←[Xtilde] 2π3/2 transition of bromocyanogen cation in the 445–560 nm region is reported. On the basis of this the electronic origin of this transition is reassigned to 18 756·20(±0·01) and 18 759·79(±0·07)cm−1 for 79BrCN+ and 81BrCN+, respectively. Vibrational assignment of the spectrum leads to consistent values for ν3 (C−Br stretch) in the upper state. In addition, a double resonance technique (stimulated emission pumping) was employed to confirm the assignment for bands to the two isotopic bromine species, and enabled the ν3 frequency in the ground state to be determined. Tentative assignments are made of bands involving the Rennet-Teller split ν2 mode excited in both the upper and lower states. Rotationally resolved spectra are reported for the 00/0, 30/1 and 30/2 and 30/120/2 bands. Analysis of these bands yields values for the ground state rotational constants. The rotational parameters determined for the different vibronic levels in the upper [Btil...

Observation of O2(b 1Σg+ → X 3Σg−) chemiluminescence from the self-reaction of isopropylperoxy radicals

Abstract We report the first direct observation of singlet oxygen product resulting from a gas phase self-reaction of alkylperoxy radicals. The evidence is a rotationally resolved O2(b 1Σg+ → X 3Σg−) chemiluminescence spectrum observed in a 12 l integrating sphere reactor containing less than 1014 isopropylperoxy radicals per cubic centimeter diluted in helium and O2 at a total pressure of 80 Pa (0.6 Torr). According to a mechanism proposed by Russell in 1957, the self-reactions of non-tertiary alkylperoxy radicals proceed by formation of a weakly bound dialkyl tetraoxide intermediate which can decompose to yield a carbonyl, an alcohol and O2. There are five energetically accessible product state potential energy surfaces. Spin conservation forbids the ground state products, but allows the formation of O2 in the 1Σg+ or 1Δg states or of T1 carbonyl. We claim these results to be strong evidence for the occurrence of Russells mechanism in the gas phase.

Oxygen atom detection using third harmonic generation

UNLABELLED Atomic oxygen has been detected using the four-wave mixing technique of third harmonic generation (THG). Ground state O((3)P(j)) atoms, generated using a microwave discharge, were monitored by focusing a 391-nm dye laser beam into the post-discharge region and observing radiation at 130 nm, near resonant with the 3s (3)S(0)(1)? 2p(4)(3)P(j) transition, produced along the laser propagation direction. The observed concentration dependent frequency shifts of the laser excitation spectrum verified the assignment of THG. Oxygen atoms were also detected in the same manner following photodissociation of NO(2). An important point for diagnostic purposes is that both the VUV intensity and peak frequency shift are dependent on O atom concentration. The relationship between these variables was explored by employing chemical titrations to establish absolute concentration. The demonstrated sensitivity is 5 x 10(13) cm(-3), but this could be significantly improved. Implications for three-photon resonant multiphoton ionization detection as well as in situ monitoring of atomic oxygen metastable states ((1)D(2) and (1)S(0)) are discussed. Third harmonic generation provides state selective and low aperture detection with sensitivity comparable to other optical methods and shows promise as a general detection technique. KEYWORDS Third harmonic generation, optical detection, four-wave mixing, oxygen, atomic detection, atomic spectroscopy, VUV radiation.

Rotational analysis of the [Btilde] 2Π3/2 ← [Xtilde] 2Π3/2 laser excitation spectrum of chlorocyanide cation

High-resolution laser excitation spectra of the [Btilde] 2Π3/2 ← [Xtilde] 2Π3/2 1 v 0 (v = 0–2) bands of the chlorocyanide cation have been recorded and rotationally analysed. The rotational constants for both 35ClCN+ and 37ClCN+ in the v″ = 0 level of the ground electronic state were determined, as well as for some levels in the excited [Btilde] state. Rotational perturbations are observed in many of these bands.

Detection of atomic nitrogen by third harmonic generation

The four‐wave mixing technique of third harmonic generation (THG) was used to detect gas‐phase atomic nitrogen atoms. Ground state N(4S03/2) atoms were produced in a microwave discharge and monitored via 120 nm radiation which was generated by a 360 nm pulsed dye laser beam focused into the post‐discharge region. The THG excitation features were blue‐shifted from the 3s 4PJ→2p3 4S03/2 atomic resonance lines. Both the frequency shift and VUV intensity were sensitive to the N(4S03/2) concentration as expected for the THG process. Absolute concentration was determined using standard flow reaction calibration and yielded a minimum sensitivity of 3×1013 cm−3. Similar THG detection schemes for atomic nitrogen metastable states (2p3 2D0j and 2p3 2P0J) are discussed. The third harmonic generation technique possesses a number of potential advantages for in situ optical diagnostics.

Characterization of the gas phase bromocyanogen cation with laser‐excited fluorescence spectroscopy

Observation of the B2Π3/2←X2Π3/2 transition of bromocyanogen cation using laser‐excited fluorescence (LEF) spectroscopy is reported. On the basis of these measurements, the vibrational frequencis of the B2Π3/2 state were confirmed and the electronic origin of the B2Π3/2←X2Π3/2 transition reassigned. Stimulated emission pumping (SEP) spectroscopy was used to verify the isotopic assignments as well as determine the X2Π3/2 (ν3) frequency. Tentative assignments are made for Renner–Teller components of transitions involving ν2 excitation in both the B2Π3/2 and X2Π3/2states. Analysis of the rotationally‐resolved spectra yielded values for the ground state rotational constants from which the Rs structure of the X2Π3/2 state was derived. Consideration of the spectroscopic parameters determined for different vibronic levels of the B2Π3/2 state indicate that some of the levels are perturbed.

Recent Developments in Studies of Open-Shell Cations Via Their Electronic Transitions

Some of our recent developments aimed at spectral characterization of open-shell cations by means of their electronic transitions will be outlined in this contribution. These involve on the one hand the use of the well established methods — laser excitation and matrix absorption spectro-scopy — to the study of the first fragment ion, C2 +, and on the other hand the application of new approaches based on stimulated emission pumping or two-colour absorption spectroscopy on mass-selected ions.