UV Spectroscopy and UV Lasers - P. Misra, M. Dubinskii (Marcel Dekker, 2002) WW.pdf - Spectroscopy - stepovy

Spectroscopy

Ultraviolet

and UV lasers

edited by

Prabhakar Misra

Howard University

Washington, D.C.

Mark A. Dubinskii

Magnon, lnc.

Reisterstown, Maryland

MARCEL

MARCELDEKKER,

NEW YORK BASEL

DEKKER

INC.

ISBN: 0-8247-0668-4

This book is printed on acid-free paper.

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Preface

Ultraviolet Spectroscopy and UV Lasers covers a range of subjects, from ul-

traviolet (UV) and vacuum ultraviolet (VUV) sources to the latest advances in

instrumentation and techniques for absorption, emission, and ﬂuorescence spec-

troscopy. The book will prove useful to scientists pursuing spectroscopy-related

research in ﬁelds as varied and diverse as optical physics and engineering, analyti-

cal chemistry, biology, and laser technology. The book can also serve as a refer-

ence text for a two-semester special topics course for upper-level undergraduate

and graduate students in these disciplines.

In the opening chapter , ‘‘Ultraviolet and Vacuum Ultraviolet Sources and

Materials for Lithography,’’ Liberman and Rothschild review radiation sources in

the UV and VUV, followed by an appraisal of the key factors governing efﬁcient

detection of UV radiation. Their overview of sources and detectors, although

presented in the context of lithographic applications, will prove useful for other

UV spectroscopic applications as well. UV optical metrology is covered under

two subheadings: measurements of bulk material properties and surfaces and

measurement of thin ﬁlm properties. Candidate optical materials for lithographic

systems are reviewed in light of their index homogeneity, birefringent properties,

and durability. An important consideration in the use of optical components for

metrological applications at UV wavelengths is the issue of large absorption coef-

ﬁcients exhibited by vapors and surface adsorbates, which is the subject of discus-

sion in the concluding segment of this chapter.

The second chapter , ‘‘Laser Optogalvanic Spectroscopy of Discharge Plas-

mas in the Ultraviolet Region’’ is coauthored by one of the editors (PM) of this

book and provides an overview of the optogalvanic effect. Owing to its sensitivity

and selectivity, the optogalvanic technique has been utilized for a wide variety

of spectroscopic applications, including analytical ﬂame spectrometry, atomic

and molecular spectroscopy, laser stabilization and calibration, and plasma diag-

nostics. Haridass, Major, Misra, and Han provide both an experimental back-

ground and a theoretical framework for reporting an extensive array of optogal-

vanic spectral transitions and selective waveforms in the UV for both neon and

argon. The mechanism associated with the generation of the optogalvanic signal

is covered in depth, and a theoretical model is developed and used to analyze

and interpret time-resolved waveforms of speciﬁc optogalvanic transitions of

neon and argon in the context of electron collisional ionization and electron colli-

sional transfer rates in a discharge medium.

In Chapter 3 , ‘‘Spectra of the Isotopomers of CO ,N 2 , and NO in the

Ultraviolet,’’ Reddy and Haridass describe the experimental recording of data

with a hollow cathode discharge tube, followed by a subsequent detailed analysis

of the electronic spectra of these species. The ultraviolet spectroscopy of these

molecules is important for the understanding and elucidation of signiﬁcant astro-

physical processes, as well as for improved understanding of associated terrestrial

environmental and combustion phenomena. A review of the relevant electronic

conﬁgurations and the general spectroscopic equations needed for a detailed spec-

tral analysis and interpretation is provided. A comprehensive historical overview

of past and present work is provided, in turn, for CO ,N 2 , and NO, followed by

presentation and analysis of UV spectra for the isotopomers of all three molecular

species. Schematic energy level diagrams illustrate the allowed spectral transi-

tions and the associated branches in detail. Precise molecular parameters charac-

terizing the various electronic states of the different species have been determined

using rigorous nonlinear least-squares ﬁtting routines and are tabulated for easy

reference.

Photoabsorption cross-section measurements in the ultraviolet and vacuum

ultraviolet are the subject of Chapter 4 . Yoshino outlines the earlier cross-section

measurements—prior to 1980—and discusses the current instrumentation avail-

able for precise cross-section measurements in the UV and VUV. A major uncer-

tainty in cross-section measurements with a single path spectrometer is in estimat-

ing the background intensity level, since it may vary with wavelength and time

during the course of the photoabsorption scan. It is also often difﬁcult to maintain

the gas pressure constant within the absorption cell, particularly for reactive and/

or absorbent species. Yoshino summarizes these difﬁculties in the context of

cross-section measurements of the photoabsorption continuum under lower reso-

lution, citing the speciﬁc example of the H 2 O molecule. He then reviews measure-

ments of cross-sections for molecular bands possessing associated ﬁne rotational

structure under superior resolution in relation to the Doppler widths of relatively

simpler molecules. Line-by-line cross-section measurements of the Schumann-

Runge bands of O 2 and their temperature dependence are then discussed. Line

and band oscillator strengths are subsequently related via the H¨nl-London factor,

whereby once the band oscillator strengths are established, cross-sections for any

temperature can in principle be determined.

In Chapter 5 , ‘‘Ultraviolet and Vacuum Ultraviolet Laser Spectroscopy Us-

ing Fluorescence and Time-of-Flight Mass Detection,’’ Lipson and Shi review

the principles and techniques of ultraviolet lasers produced by frequency doubling

in anisotropic crystals, nonresonant third harmonic generation, and two-photon

resonant four-wave sum- and difference-frequency generation in isotropic gases.

Their application focus is on single-photon excitations of supersonically jet-

cooled gas-phase molecules that are detected either via laser-induced ﬂuorescence

or by ion detection via time-of-ﬂight mass spectrometry. The authors provide a

detailed discussion of supersonic jet expansions, which have now been estab-

lished as a powerful tool for gas-phase spectroscopy, because a jet environment

is highly conducive to the production and probing of rovibrationally cold mole-

cules, free radicals, and clusters. This chapter provides practical examples of

UV and VUV laser-induced ﬂuorescence and time-of-ﬂight excitation spectra

in supersonic jets. The spectroscopy of several metal monohalide radicals and

diatomic halogens is presented in some detail. A comprehensive catalog of atomic

and molecular systems studied with VUV lasers is provided in tabular form.

The table includes a summary of the electronic states investigated by speciﬁc

spectroscopic methods for various atomic and molecular species, along with the

pertinent literature references. The use of VUV lasers in tandem with mass spec-

trometry provides unique opportunities for detection and characterization of or-

ganic molecules. The practical use of these techniques is illustrated in the context

of organic molecules (with several examples), and a detailed referenced tabula-

tion of organic molecular systems detected by nonresonant VUV ionization has

been included. The authors also provide a comprehensive bibliography of the

literature associated with the twin techniques of ﬂuorescence spectroscopy and

time-of-ﬂight mass detection.

In Chapter 6 , Cefalas and Sarantopoulou review spectroscopy and applica-

tions of diatomic and triatomic molecules assisted by laser light at 157.6 nm.

The authors begin with a description of the useful characteristics of the 157.6

nm radiation source—a molecular ﬂuorine laser—and of its utility for photo-

lithography, and then lead up to a summary of the laser’s usage for elucidating

the spectroscopy of such varied molecules as OH, O 2 , HCl, DCl, CH 3 Cl, H 2 , OCS,

H 2 O, HDO, ICN, AsF 2 ,O 3 ,CO 2 , and SiH. Spectroscopic investigations relating

to the properties of neutral or charged clusters can be accomplished in the VUV

via optical and mass spectroscopic techniques. Such approaches to study clusters

in the VUV possess two distinct advantages, namely, that systems with a large

band gap can be studied and that excitation of inner shell electrons allows ele-

ment-speciﬁc information to be collected. In this context, the VUV spectroscopy

of triatomic clusters is reviewed. It is followed by a discussion of the laser-

UV Spectroscopy and UV Lasers - P. Misra, M. Dubinskii (Marcel Dekker, 2002) WW.pdf

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