Brian J. Orr

Interferences due to easily ionised elements in a microwave-induced plasma system with graphite-furnace sample introduction

Abstract Several aspects of both enhancement and suppression of the analyte emission intensity caused by an easily ionised element (EIE) have been studied in an atmospheric pressure He microwave-induced plasma (MIP). A sequence of experiments, designed to elucidate possible mechanisms of this EIE effect, examines the following aspects: the concentration dependence of the effect for various EIEs; spatially separated vaporisation of EIE and analyte into the plasma; the effect of operating parameters upon the EIE-induced enhancement; the influence of the EIE on the excitation temperature and on the efficiency of coupling of microwave energy to the cavity. The EIE-induced suppression of emission intensity is consistent with reduced power dissipation in the plasma, due to decoupling of the plasma from the microwave power source, whereas the EIE-induced enhancement of emission intensity is best explained by a radiative energy transfer mechanism.

Spatial Emission Properties of a Microwave-Induced Plasma

The spatial dependence of emission from a microwave-induced plasma in argon has been studied. A graphite furnace atomizer was used as a means of sample introduction. Emission from metallic elements is localized to a few cm near the inlet of the discharge, but the exact position of the emission profile is element-dependent. For non-metals, such as I, a broad profile centered about the resonant cavity is found. This difference in behavior is shown to be due to the deposition of metals upon the walls of the discharge tube used to confine the plasma. The removal of analyte atoms is explained by a mechanism which involves ionization of the analyte and then radial acceleration of these ions under the influence of the inhomogeneous microwave field. This hypothesis accounts for the observed decrease in emission intensity as microwave power is increased.

Infrared-ultraviolet double-resonance studies of rotational relaxation in D2CO

Abstract Time-resolved infrared-ultraviolet double-resonance spectroscopy provides a sensitive means of studying collision-induced rotational relaxation rates and mechanisms. Relaxation of the 12 3,10 rotational state in the v 4 = 1 vibrational level of D 2 CO is found to follow electric-dipole selection rules with rates determined absolutely to be as much as five times gas kinetic.

Time-resolved infrared-ultraviolet double resonance studies of rotational relaxation in D2CO

Abstract Rotational relaxation of the ν4 = 1, J = 18, Ka = 11 state of D2CO is found to be faster than gas-kinetic and to involve multiple quantum changes in J within a single collision between D2CO molecules. The results conform to scaling laws based on |ΔJ| or the energy change |E| for the state-resolved molecule.

Mode-to-mode vibrational energy transfer in D2CO: Evidence of coriolis-enhanced rotational selectivity

Abstract Infrared—ultraviolet double resonance spectroscopy is used to demonstrate rapid collision-induced V-V transfer between the v 6 and v 4 vibrational manifolds of D 2 CO. The rate of transfer is at least gas-kinetic and is explained in terms of Coriolis coupling and rotationally specific, quasi-resonant relaxation channels

Spectrometric analysis of non-metals introduced from a graphite furnace into a microwave-induced plasma

A low-power helium microwave-induced plasma, sustained in a cylindrical TM(010) cavity, has been used with sample introduction from a graphite furnace. An end-on optical configuration was employed to monitor both atomic and ionic emission from Cl, I, S and P. The operating parameters were optimized with respect to the nature of the plasma background response, the limits of detection, and the shapes and linearity ranges of the log-log analytical working curves. Possible applications were evaluated by determining iodine in milk and analysing a multi-component mixture of sulphur compounds.

Fluorescence-detected Raman-optical double-resonance spectroscopy of glyoxal vapor

A novel Raman-optical double-resonance (ODR) technique is described in detail and applied to studies of molecular glyoxal (C2H2O2). The technique employs a pulsed excitation sequence, consisting of coherent Raman pumping of a molecular rovibrational transition followed by rovibronic probing through visible laser-induced fluorescence (LIF). The experiments demonstrate a 103-fold enhancement of sensitivity, relative to established coherent Raman spectroscopic methods, and enable individual sets of O-, P-, Q-, R-, and S-branch Raman transitions to be distinguished with high specificity under effectively collision-free conditions. For trans-glyoxal, a typical excitation sequence is X˜, v = 0, (J″, K″) → X˜, v2 = 1, (J, K) → A, v′ = 0, (J′, K′), where X˜ and A denote the ground (1Ag) and first excited (1Au) electronic singlet states, respectively, and ν2 is the symmetric CO stretching mode of vibration. There is also evidence of contributions from hot bands involving sequences in the low-frequency modes, ν7 and ν12. The Raman-ODR spectra are analyzed to yield new spectroscopic constants for the X˜, v2 = 1 vibronic level of trans-glyoxal. Variation of the time delay between Raman-excitation and LIF-probe pulses has permitted direct observation of collision-induced rotational relaxation in the ground electronic manifold of glyoxal at a rate that is ~6.5 times gas-kinetic.

Rotationally resolved infrared-ultraviolet double resonance spectroscopy in molecular D2CO and HDCO

Abstract Rotationally resolved infrared-ultraviolet double resonance (IRUVDR), consisting of sequentially excited rovibrational and rovibronic transitions sharing a common intermediate molecular level, is demonstrated. The technique employs pulsed CO2 and tunable dye lasers and is applied to the molecules D2CO and HDCO. For D2CO, infrared pumping produces 100fold population enhancement in specific rotational sublevels of the v4 = 1 level of the X 1 A 1 electronic ground state, followed by ultraviolet excitation in the 365-nm 410 band of the A 1 A 2 ← X 1 A 1 electronic system. This ultraviolet excitation occurs at a specific set of dye laser frequencies, determined by the preceding rovibrational transition, and is detected by molecular fluorescence in the visible region. Similar effects observed in HDCO involve rovibrational pumping to either the v6 = 1 or v5 = 1 levels and give rise to enhanced rovibronic transitions in the 610 and 510 bands of the A ← X system, respectively. The resulting IRUVDR spectra enable detailed spectroscopic assignments to be made and are consistent with previous results from infrared and ultraviolet absorption, laser Stark, and infrared-radiofrequency double resonance spectroscopy. Collision-induced satellite structure, arising from rotational relaxation of the intermediate rovibrational level in the IRUVDR sequence, is also reported.

Rotationally resolved Raman-optical double resonance with fluorescence detection

A pulsed excitation sequence, consisting of coherent Raman pumping of a molecular rovibrational transition followed by rovibronic probing through visible laser-induced florescence, is used in Raman-optical double resonancestudies of D(2)CO. The experiments demonstrate enhanced sensitivity, with respect to conventional coherent Raman spectroscopy, and enable individual O- and P-branch Raman transitions to be distinguished with high specificity under effectively collision-free conditions.

Dissociative excitation transfer from sulphur hexafluoride following pulsed infrared irradiation

Abstract Mixtures of SF 6 with various simple hydrocarbons are irradiated at ≈2 torr by CO 2 laser pulses of power density 10 8 W cm -2 , yielding transient visible luminescence from products of hydrocarbon dissociation. Stepwise multiple photon excitation of SF 6 molecules is shown to initiate the rapid sequence of events leading to hydrocarbon dissociation. The most probable mechanism consists of infrared photodissociation of SF 6 , the F atoms from which react with the hydrocarbon, yielding C 2 and CH chemiluminescence.