The precise determination of relative abundances of ultra trace isotopes in the range below 10^-9 is of importance for a wide spectrum of applications in fields like environmental protection, cosmo-chemistry, bio-medical tracer studies or geological and geo-chronological investigations. The necessary high isotopic selectivity, rather complete isobaric suppression and good overall efficiency for these investigations is provided by high-resolution resonance ionization mass spectrometry. Multi-step continuous wave laser excitation and ionization using diode lasers at a compact quadrupole mass spectrometer has been optimized to become a powerful and reliable experimental method, which is just becoming competitive to accelerator mass spectrometry. With this technique we have performed measurements with high isotopic selectivity above 10¹⁰ on the isotopes ⁹⁰Sr and ⁴¹Ca while further studies concern Gd and Pu isotopes. In all cases extensive atomic spectroscopic investigations were required as basis for analytical studies.

The performance of the Resonance Ionization Mass Spectrometry (RIMS) system at the National Institute of Standards and Technology (NIST) has been compared to a similar system at Pacific Northwest National Laboratory (PNNL). Efficiency and selectivity measurements were performed with both systems and compared to conventional thermal ionization mass spectrometry (TIMS). Determination of the ¹³⁵Cs / ¹³⁷Cs ratio was performed using single-resonance excitation 6s ²S_1/2 (F equals 4) to 6p ²P_3/2 (F equals 5) with an extended cavity diode laser followed by photoionization with the 488 nm line of an argon ion laser. Optical selectivity of more than 2 orders of magnitude against stable ¹³³Cs was attained for ¹³⁵Cs and ¹³⁷Cs for both systems with an overall selectivity of 10⁹ for the PNNL system and 10⁸ for the NIST system. Overall efficiencies of 2x10^-6 and 5x10^-7 were measured for the PNNL and NIST systems respectively. Measurements to determine the chronological age of a nuclear burn-up sample have been performed using both RIMS systems as well as TIMS.

We have developed a novel version of collinear fast beam laser spectroscopy based on particle detection. The sample to be analyzed is loaded into an ion source and a mass separated fast ion beam is produced. In it we can detect with high efficiency the various krypton isotopes, and in particular the long lived rare isotope ⁸⁵Kr by observing the optical hyperfine structure spectrum. The technique utilizes cascade two-step excitation to pump metastable krypton atoms to a high-lying Rydberg level. The metastable atom level is effectively populated from the Kr ion beam by a near resonance charge exchange reaction with rubidium vapor. Thereupon the Rydberg atoms are field-ionized and the resulting signal ions, which have been at resonance with two laser frequencies, are detected. The following transitions are used in the two-step excitation process: 5s² [3/2]₂ yields 5p² [5/2]₃ yields 29d² [7/2]₄. The technique has been successfully applied to all the stable krypton isotopes, and to ⁸⁵Kr, but not yet to ⁸¹Kr. After the Kr ions are emitted from the ion source the overall detection efficiency is to date 8%. The selectivity is at the one part in 10¹⁰ level and the sensitivity at a few hundred ions/s. Obvious further improvements are possible and are outlined. A similar excitation scheme can also be applied to the other noble gases including radon and to their rare isotopes.

We report the first magneto-optical trapping of radioactive ¹³⁵Cs and ¹³⁷Cs and a promising means for detecting these isotopes to ultrasensitive levels by using a magneto-optical trap (MOT) coupled to a mass separator. A sample containing both isotopes was placed in the source of a mass separator, ionized, mass separated, and implanted in a Zr foil within the MOT cell. After implantation, atoms were released from the foil by inductive heating and then captured in a MOT that used large diameter beams and a dry-film-coated cell to achieve high trapping efficiency. MOT fluorescence signals were measured for trapped-atom numbers from 10⁴ to 10⁷ and were found to increase linearly with the number of atoms implanted in the foil. The slope of signal versus number implanted was equal for each isotope to within 4%, signifying our ability to measure ¹³⁷Cs/¹³⁵Cs ratios to within 4% for MOT signal levels exceeding that associated with our present detection limit of 4000 trapped atoms. The MOT-based detection scheme was shown capable of suppressing interference from stable ¹³³Cs by more than seven orders of magnitude. Including an isotopic selectivity of 10⁵ of the mass separator, the overall suppression of ¹³³Cs in the case of detecting either ¹³⁵Cs and ¹³⁷Cs is expected to exceed 10¹². At present, the overall sample detection sensitivity is less than one million atoms.

An experiment is described in which 10⁸ CaH molecules have been confined in a magnetic trap at a temperature of 400 mK. The elastic scattering cross section of CaH on ³He was measured to be σ_elequals(1.5±0.6)×10^-14cm². A lower bound of Γ_rotational⩾10^-15cm³s^-1 was placed on the rotational elastic collision rate coefficient, and an upper bound of Γ_ν<10^-15 cm³s^-1 was placed on the vibrational relaxation rate coefficient. The rate coefficient for spin changing collisions was measured to be, within an order magnitude, Γ _Zequals10^-17 cm³ s^-1.

We present a systematic analysis of ultra-sensitive molecular detection methods based on an optical cavity and frequency modulation spectroscopy. Our goal is towards the improvement of the limiting attainable performance by the choice of optical configuration, sample gas pressure, laser mode size, laser intensity, and detection method was discussed. The application of sensitive detection techniques emphasized here is for laser frequency stabilization, leading to better optical frequency standards and clocks.application of sensitive detection techniques emphasized here is for laser frequency stabilization, leading to better optical frequency standards and clocks.

Using the Berkeley medium-resolution pulsed cavity ringdown spectrometer we have observed the HCl stretch spectrum of the HCl-H2O dimer and DCl stretch mode of the completely deuterated analog. The shifts from the free HCl stretch are consistent with theory and Ar matrix isolation work. Rotational structure was obtained for the deuterated cluster and spectroscopic constants for both chlorine isotopomers determined. Cluster number densities were determined to be slightly lower than found for the pure water dimer under similar conditions.

Modulation Spectroscopy is a sensitive, convenient, versatile and cost-effective method for monitoring gaseous species and for obtaining quantitative information about molecular collision dynamics through precise measurements of the absorption lineshape function. Even slight perturbations in the lineprofile can be measured precisely, and because these perturbations are directly linked to changes in physical conditions of the sampled target, one obtains very precise non-intrusive measurements of these parameters. Over the last few years, we have extended this technique to the regime of higher harmonic detection and demonstrated that, in many cases, one obtains a higher precision by using an optimal harmonic detection order higher than the commonly used second harmonic. Experimental and theoretical results have been presented. In this paper we use the principles of Information Theory developed by Shannon to describe the information content in modulation spectroscopic signals. A simple argument is used to show that information that may otherwise be lost because of distortion can be recovered by derivative like techniques, such as those used in low frequency modulation spectroscopy. Experimental results obtained for the resolution of overlapping lines of disparate strength are discussed.

The transverse force of an optical trap is usually measured by equating the trapping force to the viscous drag force applied to the trapped particle according to Stokes' Law. Under normal conditions, the viscous drag force on a trapped particle is proportional to the fluid velocity of the medium. In this paper we show that an increase of particle concentration within the medium affects force measurements. In order to trap the particle, 1064 nm light from a Nd:YVO₄ laser was brought to a focus in a sample slide, of thickness around 380 microns, by using an inverted Zeiss microscope objective, with NA equals 1.3. The slide was filled with distilled water containing 6 micron diameter polystyrene spheres. Measurements were taken at a fluid velocity of 0.75 microns/sec, achieved by moving the sample stage with a piezo-electric transducer whilst a particle was held stationary in the trap. The laser power required to hold a sphere at different trap depths for various concentrations was measured. Significant weakening of the trap was found for concentrations >0.03% solids by weight, becoming weaker for higher trap depths. These results are explained in terms of aberrations, particle-particle interactions and distortion of the beam due to particle-light interactions.

In this work we describe preliminary experiments in which we have used ultra-sensitive fluorescence microscopy to observe the dynamics of individual enzyme molecules acting upon a substrate. The enzyme, (beta) -galactosidase from E.coli, is specifically immobilized onto a glass substrate while maintaining its functionality. The immobilized protein degrades a fluorogenic substrate to produce a fluorescent product, whose generation can be observed in real time. Individual copies of (beta) -galactosidase can be observed for many minutes, allowing the measurement of a large number of successive substrate turnover events. A rudimentary analysis of these turnovers using autocorrelation functions is presented, and a strong heterogeneity in reaction rates between different molecules is observed. In addition, the challenges inherent in successful surface immobilization of proteins for single-molecule experiments are discussed.

Fluorescence lifetime measurement of organic fluorophores is a powerful tool for distinguishing molecules of interest from background or other species. This is of interest in sensitive analysis and Single Molecule Detection (SMD). A demand in many applications is to provide 2-D imaging together with lifetime information. The method of choice is then Time-Correlated Single Photon Counting (TCSPC). We have devloped a compact system on a single PC board that can perform TCSPC at high throughput, while synchronously driving a piezo scanner holding the immobilized sample. The system allows count rates up to 3 MHz and a resolution down to 30 ps. An overall Instrument Response Function down to 300ps is achieved with inexpensive detectors and diode lasers. The board is designed for the PCI bus, permitting high throughput without loss of counts. It is reconfigurable to operate in different modes. The Time-Tagged Time-Resolved (TTTR) mode permits the recording of all photon events with a real-time tag allowing data analysis with unlimited flexibility. We use the Time-Tag clock for an external piezo scanner that moves the sample. As the clock source is common for scanning and tagging, the individual photons can be matched to pixels. Demonstrating the capablities of the system we studied single molecule solutions. Lifetime imaging can be performed at high resolution with as few as 100 photons per pixel.

The use of micro droplets as a medium for sensitive detection in fluorescence-based immunoassays has been explored in two contexts. The competitive immuno-reaction of a pesticide hapten, esfenvalerate, with its antibody was performed in micro droplets generated by a vibrating orifice aerosol generator system with a 10-micrometers diameter orifice. Fluorescence from Rhodamine 6G was excited by the second harmonic of a Nd:YAG laser and detected by a 1/8 m imaging spectrograph with a 512 X 512 thermoelectrically cooled, charged-coupled device (CCD) camera. The conjugate of esfenvalerate with rhodamine exhibited similar fluorescence to that of pure rhodamine 6G. When anti-esfenvalerate antibodies were added to the droplets, the fluorescence decreased due to quenching. When a sample of esfenvalerate was added to the droplets, esfenvalerate and esfenvalerate- rhodamine conjugate competed for binding with the anti- esfenvalerate antibody. The release of the conjugated rhodamine from the antigen-antibody complex allowed the fluorescence signal to recover. The assay in a picoliter droplet sample was shown to enable detection down to approximately 0.1 nM. Micro droplets also exhibit strong cavity-dependent optical behavior that gives rise to lasing action. Lasing from Rhodamine fluorescence was quenched by the addition of a second dye, oxonol, that absorbs in the spectral region where the Rh 6G fluorescence peaked. A very strong impact on the droplet resonances was observed, leading to the possible use of quenching as an assay for oxonol-labeled haptens.

It is long known that the fluorescence properties of dyes are significantly changed when closely approaching a metal surface. This is caused by the strong electromagnetic coupling between evanescent modes of the fluorophores' radiation and the electron gas of the metal. In extreme cases, the fluorescence is completely quenched by the metal. However, under favorite conditions, the dye/metal interaction can strongly enhance the fluorescence instead of quenching it. In the present paper, the fluorescence properties of dye within a spherical nanocavity is theoretically studied, and it is shown that the fluorescence brightness and photostability can be enhanced by several orders of magnitude.

A key technique of detecting the ultra-weak photon emission from biological system (UPE) is to change the light signal of an extremely weak level into electric signal of a considerable level when the photo-electric detecting system were be applied. This paper analyzed the difficult for detecting the ultra-weak photon emission from biological system (UPE) mainly is in the absence of high sensitivity detector in UV-visible-infra spectra region. An experimental setup for testing UPE in different spectral region was designed. Using the experimental setup the test data of different several spectral regions from 300 nm to 1060 nm has were tested. The test result show the UPE of living biological system exists in wide spectra region from UV- visible to infrared.

Lightsources employing quasiphasematched (QPM) nonlinear materials have demonstrated unique attributes for chemical sensing in the near- to mid-infrare spectral range (1 - 5 micrometers ). The advent of patterned-growth GaAs allows the first practical extension of QPM materials to operation in the long-wave IR (5 - 12 micrometers ). That wavelength range is particularly attractive for chemical sensing because it contains an atmospheric window, many molecular groups absorb there at distinct frequencies, and their absorptions tend to be strong relative to those in the near- and mid-IR. Here, the application of orientation-patterned GaAs (OPGaAs) for use in a continuous wave (cw) difference frequency spectrometer is described. The outputs of two external- cavity diode lasers operating in the 1.3 and 1.5 micrometers telecom bands are mixed in a OPGaAs crystal, producing tunable radiation at wavelengths near 8 micrometers . The application of the source to the measurement of a water vapor rovibrational absorption line is presented.