The technique of Resonance Ionization Spectroscopy (RIS) is being extended to develop a means for counting individual atoms of a selected isotope of a noble gas. In this method, lasers are used for RIS to obtain atomic species (Z) selectivity and a small quadrupole mass spectrometer provides isotopic (A) selectivity. A progress report on the objective of counting each atom of a particular isotope of a noble gas is given. Resonance ionization spectroscopy and its use for the detection of single atoms has been reviewed.' More recently, our efforts at ORNL have turned to the problem of direct counting of noble gas atoms2,3,4 as an alternative to decay counting of particular isotopes of noble gas species. For broader applications, the ORNL group is trying to develop a facility for counting a few rare gas atoms of a given isotopic variety in a sample. The detection of a small number of 81Kr atoms (<1000) is very important for groundwater dating, polar ice-cap dating, and nuclear waste disposal applications, and solar neutrino research. The ultimate goal is to count a small number (e.g., 100 to 1,000) of selected atoms having mass number A, even when mixed with 1012 or more atoms having mass number ± 1. The experimental schematic is shown in Figure 1. The concept for counting noble gas atoms with isotopic selectivity is to utilize a laser for ionizing atoms of a selected atomic

The mass spectrometric determination of accurate atomic isotope ratios is a task greatly complicated by the presence of isobaric (same mass) interferences. While there exist classical techniques for reducing such interferences, such as ultrahigh resolution mass spectrometry, these solutions often break down in the face of particular technical difficulties, such as the determination of very large isotope ratios or very small samples.

The technique of resonance ionization mass spectrometry (RIMS) has been successfully applied to the measurement of isotope ratios for several elements. In the case of neodymium and samarium, isobaric interference was removed at isotopic masses 144, 148 and 150. Isotope ratios were measured for these two elements in an equimolar mixture, with a significant improvement in accuracy compared to conventional thermal ionization mass spectrom-etry. In addition, RIMS was applied to the elements uranium and plutonium in order to eliminate isobaric interferences at mass 238 and potentially at mass 241 (due to americium). Recent results are presented for measurements made on nanogram size samples loaded on resin beads.

Recent demonstrations of the extremely high efficiency and selectivity of multiphoton resonance ionization(l-5) have stimulated interest in combining this ionization mode with the mass selectivity of mass spectrometry to produce a resonance ionization mass spectrometer (RIMS) with superior isotopic selectivity and sensitivity. Such a measurement system can be functionally categorized into tour components: atomization, ionization, ion manipulation, and ion detection and labeling. It is the purpose of this paper to explore systematically the potential of RIMS as realized with a magnetic sector mass spectrometer.

Recently, multiphoton resonance ionization (MPRI) has been coupled with energetic ion bombardment to yield a highly efficient and selective tool for solids analysis. Although this method promises to yield sub-ppb analysis for some materials, there are a number of experimental factors which will utlimately limit the analytical sensitivity of the technique. Among these factors are a) duty cycle, b) primary ion current, c) sputter yield, d) fraction of ejecting particles which are ionizable, and e) detection efficiency. This paper discusses the origin of these factors and their influence on the use of MPRI for trace analysis of solids.

This paper describes a new technique, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS)t, for ultrasensitive elemental analysis of solid samples. SIRIS combines resonance ionization spectroscopy and ion beam sputtering to provide analyses for all the elements except helium and neon with predicted sensitivities down to 1 part in 1012 in routine analysis, and greater for special uses. Basic concepts of this technology and new results in the development of the new SIRIS process and apparatus are presented.

We have developed a cryogenic He-jet system which efficiently transports radioactive atoms produced on-line at the Argonne National Lab Tandem-Linac Accelerator away from the production region and forms them into a cool atomic beam. This atomic beam will be probed with high sensitivity laser spectroscopy using the photon burst method. The ultimate goal of this work is to determine the sizes, shapes, and magnetic moments of short-lived nuclei through their atomic hyperfine structure. Preliminary measurements with the He-jet system and the adaption of the photon burst method to this new geometry are described.

Using a single-clipped digital correlator, the burst of photons resulting from laser induced resonance fluorescence of a single atom as it passes through a laser beam can be recorded and its transit-time measured. By averaging many fluorescent atoms, the transport characteristics of these atoms in a gaseous system, such as diffusion, flow velocity, and chemical reaction rates can be conveniently determined by photon-burst correlation spectroscopy.

A resonance fluorescence method is demonstrated whereby the density of an atomic beam can be determined reliably. This determination is based on the distribution of time intervals between detected photons, and is made without independent knowledge of excitation cross sections, laser beam intensity, or photon counting efficiency.

A fiber optic light collector has been constructed in order to efficiently guide photons from a long narrow source onto the face of a photomultiplier. We present here a brief review of the technique of collinear laser-fast atom beams and describe the performance of this new collector in such a collinear system.

Fluorescence light is used in spectroscopic experiments on stored ions. We discuss applications in (1) high resolution microwave and rf/optical double resonance spectroscopy, (2) single ion detection, (3) mass spectroscopy and (4) studies of stored ion clouds which exhibit properties of strongly coupled plasmas.

A sheath flow cuvette was evaluated in laser-induced fluorescence determination of aqueous rhodamine 6G. A detection limit of 18 attograms was obtained within a one-second signal integration time. The concentration detection limit was 8.9 x 10-14 mole per liter. An average of one-half rhodamine 6G molecule was present within the 11 pL excitation volume. However, dUring the signal integration time a total of 22,000 analyte molecules passed through the excitation region in a 0.42 microliter volume. The biomedical technique of flow cytometry has been used to study the fluorescence and light scatter properties of biological cells and cellular components.1 The hydrodynamic focusing property of the sheath flow cuvette employed in flow cytometry provides a well designed flow chamber for laser-induced fluorescence analysis of small volume samples. The sheath flow cuvette has been applied as a laser-induced fluorescence detector in high performance liquid chromatography and flow injection analysis.2-4 A tightly focused laser beam was used in those experiments to define an excitation volume of several nanoliters. In the present report, the performance of the sheath flow cuvette is considered for fluorescence analysis in excitation volumes of several picoliters.5 The sample cell is shown in Figure 1.

The detection of induced polarization changes can improve the sensitivity of a number of nonlinear spectroscopic techniques. We discuss in detail the technique of Doppler-free polarization spectroscopy to illustrate the basic features and the sensitivity of this approach.

Direct absorption. measurements using Frequency. Modulation. Spectroscopy (FMS) have been made. Simple calibration is possible and DC absorptions as small as 1.4x10-4 may be detected. By modulating the absorption photochemically, the sensitivity may be extended to a lower limit of 1.5x10-7. The radicals HCO1 and ND4 have been studied using this method.

The progress over the last few years in the field of sub-Doppler saturated absorption spectroscopy has been greatly assisted by the development of new techniques for increasing sensitivity1-5. For many laboratory situations it is now routinely possible to achieve signal-to-noise ratios and sensitivities very near the quantum limit imposed by the fundamental statistical fluctuations (shot noise) of the probe laser beam.

Detection systems with high sensitivity are of interest in all fields of science and engineering. No exception to this generality is the desire to be able to detect small absorptions or phase shifts of an optical beam, whether that be for detection of weak absorp tion of dilute atomic or molecular systems or for detecting small length changes in optical cavities for gravity wave research, etc. The past 15 years have seen the development of a number of good techniques for enhancing sensitivity in the detection of such perturbations to an optical beam. Almost invariably the high sensitivity detectors are laser-based systems, for example: direct subtraction of intensity fluctuations1 removal of intensity fluctuations by active control,2 laser intracavity absorption,3 derivative spectroscopy, polarization spectroscopy,4 intermodulation techniques,5 interferometric spectroscopy,6 polinex,7 heterodyne-detected Raman spectroscopy,8 etc. There are other techniques with high sensitivity which do not observe the optical beam directly but rather observe the effects of absorption from the optical beam, for instance fluorescence detection,9 optogalvanic spectroscopyl10 and opto-acoustic spectroscopy.11 Most of the methods, by some means, try to suppress the background noise which is not inherent on the signal.

We report an application of two saturated absorption techniques, saturation spectroscopy and polarization spectroscopy to low density measurements in sodium vapor. The sensitivity we achieved using the resonance lines D1 and D2, was enough to detect a minimum number of a few thousand atoms. We discuss principal limits of such experiments and the benefits of both of the techniques.

For several decades optical emission spectroscopy has been the only technique available for the investigation of electronic spectra and structures of molecular ions.1-3 The inherently strong emission produced by electronic transitions at visible and ultraviolet wavelengths has been critically important to such studies of charged species, because the chemical reactivity and Coulomb repulsion of ions limit their steady state densities to extremely low levels (typically 1 ppm of the total gas density). The sensitivity of optical emission techniques is, however, compromised by several other factors, including a rather low resolution compared to that attainable in laser applications, the strong interference from emissions of the far more abundant neutral molecular species, the requirement that a sizeable fraction of the ions be electronically excited, and the unavailability of ro-vibronic population information on the lower state of the transition. Nevertheless the majority of all rotationally resolved electronic spectra for molecular ions observed to date have been detected and measured by optical emission spectroscopy, and without this ground-work the development of more sophisticated laser techniques would have been greatly hampered.

The application of low absorption measurements to dilute solute determination requires specific instrumental characteristics. The use of laser intracavity absorption and thermal lens calorimetry to measure concentration is shown. The specific operating parameters that determine sensitivity are delineated along with the limits different measurement strategies impose. Finally areas of improvement in components that would result in improve sensitivity, accuracy, and reliability are discussed. During the past decade, a large number of methods have been developed for measuring the light absorbed by transparent materials. These include measurements on gases, liquids, and solids. The activity has been prompted by a variety of applications and a similar variety of disciplines. In Table 1 some representative examples of these methods is shown along with their published detection limits.1 It is clear that extraordinarily small absorbances can be measured. Most of the methods can be conveniently divided into two groups. These groups are those that measure the transmission of the sample and those that measure the light absorbed by the sample. The light absorbed methods are calorimetric in character. The advantages and disadvantages of each method varies depending on the principal application for which they were developed. The most prevalent motivation has been to characterize the bulk optical properties of transparent materials. Two examples are the development of extremely transparent glasses for use as fiber optic materials and the development of substrates for high power laser operation.

Chromatographic detectors based on the thermal lens and related photothermal deflection effects are discussed. A pump/probe thermal lens detector is used to monitor HPLC effluents with 1 μA sensitivity. A photothermal deflection system is used as a densitometer in a scanned HPTLC system. Picogram sensitivity is observed. Prospects for practical devices are discussed.

Techniques are described for obtaining vibrational resonance Raman spectra of dilute molecules in solution (parts per million or less) with temporal resolution of 10-8 to 10-11 seconds. These time-resolved resonance Raman (TR3) techniques are extremely valuable in elucidating the structures of short-lived molecules in chemical reactions or in condensed-phase photophysics. Applications of TR3 to the study of electronically excited states, particularly those of metal ion complexes which convert light into chemical energy, are described.

The conjunction of laser induced molecular ionization with mass spectrometry is particularly fortuitous since it enables us to learn more about the former phenomenon by exploiting the latter technique while at the same time we may be able to improve the latter technique by exploiting some of the unique properties of the former phenomenon. One of the most significant features is the potential impact that laser ionization mass spectrometry may have in analytical chemistry. In order to better measure this, we have investigated the ionization efficiency of a variety of organic compounds. The combination of a capillary column gas chromatograph with a laser ionization mass spectrometer is found to be an ultrasensitive and selective method of chemical analysis. Polyaromatic hydrocarbons can be detected at the multifemtogram level with parts per trillion sensitivity. In some cases previously unresolvable isomers are readily distin-guished. Complementary laser photoelectron experiments have been conducted. While these were originally designed to elucidate ionization and mass spectral fragmentation mechanisms, they are now generating a vast array of new ion spectroscopic data. Our recent results in the previously unexplored area of UV laser induced surface ionization are discussed and their relevance to mass spectrometry is considered.

Among the many techniques that benefited from the developement of laser technology, two examples are the improved detectability in optical rotation and in refractive index meas-urements. The former is an interesting physical property because it is usually associated with biological activity. The latter is useful because it is a universal physical property. In conjunction with liquid chromatography, one can take advantage of the improved detectability to study molecules that are present in small quantities in fairly complex samples, such as biological fluids and fossil fuels. The high degree of collimation of the laser allows better levels of extinction to be achieved between crossed polarizers. The amount of scattered light, and thus the amount of depolarization, can be greatly reduced. This then allows small rotations of the polarization axis of light to be detected. A detectability of 1.5 x 10-5 degrees can be achieved. The high monochromaticity of the laser brings about new applications of Fabry-Perot interferometry. The interference fringes can be used to monitor refractive index changes inside the interferometer. In conjunction with a flow cell interfaced to a liquid chromatograph, a detectability of 4 x 10-9 RI units is possible.

Several new laser-based detection methods for use in combustion diagnostics are described. Spatially resolved saturated absorption spectroscopy and optical Stark modulation spectroscopy provide two techniques for obtaining point resolution in absorption measurements; these methods are demonstrated in studies of atomic sodium aspirated into a flame. Resonant multiphoton optogalvanic spectroscopy can be used to observe radicals in flames that are extremely difficult to detect with conventional optical techniques; studies of atomic hydrogen and atomic oxygen in flames are presented.