Probing c60.

Experiments involving the laser vaporization of graphite have indicated that one particular duster of carbon, C(60), is preeminently stable; this special stability may be evidence that C(60) can readily take the form of a hollow truncated icosahedron (a sort of molecular soccerball). If true, this structure for C(60) would be the first example of a spherical aromatic molecule. In fact, because of symmetry properties unique to the number 60, it may be the most perfecty spherical, edgeless molecule possible. Its rapid formation in condensing carbon vapors and its extreme chemical and photophysical stability may have far-reaching implications in a number of areas, particularly combustion science and astrophysics. For these reasons C(60) and other dusters of carbon have continued to be the subject of intense research. This article provides a short review of the many new experimental probes of the properties of C(60) and related carbon dusters.

Probing chemical reactions: evidence for exploration of an excited potential energy surface at thermal energies.

The reaction K + NaBr --> KBr + Na is probed during the reactive collision by a continuous wave laser tuned to frequencies not resonant with excitation in either reagents or products. Transient [K..Br..Na] absorbs a laser photon giving [K..Br..Na](*), which can decompose to Na(*) + KBr. Emission from excited Na(*) at the sodium D lines provides direct evidence of laser absorption during the reaction. Different excitation spectra were observed, depending on which sodium D line was monitored. This difference is explicable if, in the absence of the laser, the reaction flux partially bifurcates to a second potential energy surface during the reaction.

C60 buckminsterfullerene high yields unraveled.

Recently Irle, Morokuma, and collaborators have carried out a series of quantum chemical molecular dynamics simulations of carbon clustering. The results of these computer experiments are that carbon clusters of size greater than 60 atoms are rapidly formed, anneal to giant fullerenes, and then these fullerenes shrink. The simulation could not be carried to long enough times for the shrinking to reach C60, but they propose reasonably that this shrinking process ultimately forms buckminsterfullerene. However, these simulations do not reveal the force driving the shrinking process. Here, this driving force for shrinking is found to be reactions in which C2 is swapped between fullerenes. The key element is that for typical fullerenes the equilibrium constants for such C2 interchanges are near unity, resulting in expansion of the breadth of the fullerene distribution in an annealing process. When fullerenes of 60 or 70 atoms are populated by shrinking, they fall into the local energy minimum of buckminsterfullerene or D5h C70. This simple mechanism accounts for the high yields (>20%) of buckminsterfullerene that can be achieved in pure carbon systems.

Chemical sensing with pulsed QC-DFB lasers operating at 15.6 micrometers.

Pulsed thermoelectrically cooled QC-DFB lasers operating at 15.6 micrometers were characterized for spectroscopic gas sensing applications. A new method for wavelength scanning based on repetition rate modulation was developed. A non-wavelength-selective pyroelectric detector was incorporated in the sensor configuration giving the advantage of room-temperature operation and low cost. Absorption lines of CO2 and H2O were observed in ambient air, providing information about the concentration of these species.

Spectroscopic detection of biological NO with a quantum cascade laser.

Two configurations of a continuous wave quantum cascade distributed feedback laser-based gas sensor for the detection of NO at a parts per billion (ppb) concentration level, typical of biomedical applications, have been investigated. The laser was operated at liquid nitrogen temperature near lambda = 5.2 microns. In the first configuration, a 100 m optical path length multi-pass cell was employed to enhance the NO absorption. In the second configuration, a technique based on cavity-enhanced spectroscopy (CES) was utilized, with an effective path length of 670 m. Both sensors enabled simultaneous analysis of NO and CO2 concentrations in exhaled air. The minimum detectable NO concentration was found to be 3 ppb with a multi-pass cell and 16 ppb when using CES. The two techniques are compared, and potential future developments are discussed.

Absorption spectroscopy with quantum cascade lasers.

Novel pulsed and cw quantum cascade distributed feedback (QC-DFB) lasers operating near lambda=8 micrometers were used for detection and quantification of trace gases in ambient air by means of sensitive absorption spectroscopy. N2O, 12CH4, 13CH4, and different isotopic species of H2O were detected. Also, a highly selective detection of ethanol vapor in air with a sensitivity of 125 parts per billion by volume (ppb) was demonstrated.

IR kinetic spectroscopy investigation of the CH4 + O(1D) reaction.

The branching of the title reaction into several product channels has been investigated quantitatively by laser infrared kinetic spectroscopy for CH(4) and CD(4). It is found that OH (OD) is produced in 67 +/- 5% (60 +/- 5%) yield compared to the initial O((1)D) concentration. H (D) product is produced in 30 +/- 10%(35 +/- 10%). H(2)CO is produced in 5% yield in the CH(4) system (it was not possible to measure the CD(2)O yield in the CD(4) case). D(2)O is produced in 8% yield in the CD(4) system (it was not feasible to measure the H(2)O yield). The ratio of the overall rate constant of the CD(4) reaction to the overall rate constant of the O((1)D) + N(2)O reaction was determined to be 1.2(5) +/- 0.1. A measurement of the reaction of O((1)D) with NO(2) gave 1.3 x 10(-10) cm(3) molecule(-1) s(-1) relative to the literature values for the rate constants of O((1)D) with H(2) and CH(4). Hot atom effects in O((1)D) reactions were observed.

Design considerations of an infrared spectrometer based on difference-frequency generation in AgGaSe(2).

The availability of new nonlinear optical materials such as AgGaS(2) and AgGaSe(2) and improvements in compact, tunable, pulsed and continuous-wave (cw) solid-state pump lasers now make it possible to generate tunable, infrared narrow-band coherent radiation over a wide wavelength range (4-18 µm) by means of difference-frequency generation (DFG). This article describes the wavelength and outputpower characteristics of a tunable infrared source based on AgGaSe(2) and certain proven cw near-infrared pump sources for application to high-resolution spectroscopy.

Multiwavenumber linearized diode laser spectra by overlapping frequency scans.

A method for producing diode laser spectroscopy scans which are several wavenumbers long, linear in frequency, and readily and accurately calibrated from reference spectra is described. The laser itself is current scanned under computer control over short segments (as is normally done) and such overlapping segments are linearized and pieced together to provide the final scan.

Detection of CO in air by diode-pumped 4.6-microm difference-frequency generation in quasi-phase-matched LiNbO(3).

Detection of CO, N(2)O, and CO(2) in ambient air was performed with a room-temperature cw IR source based on difference-frequency generation in periodically poled LiNbO(3). The source was pumped by a seeded highpower GaAlAs amplif ier at 860 nm and a diode-pumped monolithic Nd:YAG ring laser at 1064 nm. The IR output was tunable between 2160 and 2320 cm(-1) without crystal rotation. The CO detection sensitivity is extrapolated to 5 ppb m/ radicalHz if limited by IR intensity noise.

Continuous-wave tunable 8.7-microm spectroscopic source pumped by fiber-coupled communications lasers.

Tunable narrow-band cw difference-frequency generation at 8.7 microm was demonstrated in silver gallium selenide (AgGaSe(2)) at room temperature. The crystal was pumped by an injection-seeded Er/Yb-codoped fiber amplifier at 1.554 microm and a fiber-coupled diode-pumped monolithic ring Nd:YAG laser at 1.319 microm. The difference-frequency output was used for high-resolution spectroscopy of sulfur dioxide (SO(2)).

Quartz-enhanced photoacoustic spectroscopy.

A new approach to detecting a weak photoacoustic signal in a gas medium is described. Instead of a gas-filled resonant acoustic cavity, the sound energy is accumulated in a high- Q crystal element. Feasibility experiments utilizing a quartz-watch tuning fork demonstrate a sensitivity of 1.2x10(-7) cm(-1) W/ radicalHz . Potential further developments and applications of this technique are discussed.

Computer control of broadly tunable lasers: conversion of a color center laser into a high resolution laser spectrometer.

The conversion of a commercial color center laser (Burleigh FCL-20) into a computer-controlled high resolution laser spectrometer is described. The techniques and methods developed are generally useful for the control of any broadly tunable cw laser. Superposition of several kinds of computer-controlled feedback upon a table look-up (feed-forward) procedure is used for each of the control elements. This feedback removes the coupling between the tuning elements and eliminates the effects of the nonideal properties of the control elements such as PZT hysteresis and creep. The resulting spectrometer is capable of acquiring high resolution (instrumentally limited by free running laser jitter to ~3 MHz) spectra which can be precisely measured (~0.0003 cm(-1)). The spectrometer has routinely made continuous single-mode scans of 30,000 points separated by 10 MHz covering 10 cm(-1).

Mid-infrared difference-frequency generation source pumped by 1.1-1.5 micrometer dual-wavelength fiber amplifier for trace-gas detection.

Continuous-wave mid-infrared radiation near 3.5 micrometers is generated by difference-frequency mixing of the output of a compact 1.1-1.5 micrometer dual-wavelength fiber amplifier in periodically poled LiNbO3. The diode side-pumped amplifier is constructed with double-cladding Yb-doped fiber followed by single-mode Er/Yb codoped fiber. Output powers of as much as 11 microW at 3.4 micrometers are obtained, and spectroscopic detection of CH4 and H2CO is demonstrated.

Room-temperature mid-infrared laser sensor for trace gas detection.

Design and operation of a compact, portable, room-temperature mid-infrared gas sensor is reported. The sensor is based on continuous-wave difference-frequency generation (DFG) in bulk periodically poled lithium niobate at 4.6 mum, pumped by a solitary GaAlAs diode laser at 865 nm and a diode-pumped monolithic ring Nd:YAG laser at 1064.5 nm. The instrument was used for detection of CO in air at atmospheric pressure with 1 ppb precision (parts in 10(9), by mole fraction) and 0.6% accuracy for a signal averaging time of 10 s. It employed a compact multipass absorption cell with a 18-m path length and a thermoelectrically cooled HgCdTe detector. Precision was limited by residual interference fringes arising from scattering in the multipass cell. This is the first demonstration of a portable high-precision gas sensor based on diode-pumped DFG at room temperature. The use of an external-cavity diode laser can provide a tuning range of 700 cm(-1) and allow the detection of several trace gases, including N(2) O, CO(2), SO(2), H(2) CO, and CH(4).

Ammonia Detection by use of Near-Infrared Diode-Laser-Based Overtone Spectroscopy.

We describe a portable diode-laser-based sensor for NH(3) detection using vibrational overtone absorption spectroscopy at 1.53 mum. Use of fiber-coupled optical elements makes such a trace gas sensor rugged and easy to align. On-line data acquisition and processing requiring <30 s can be performed with a laptop PC running LabVIEW software. The gas sensor was used primarily for NH(3) concentration measurements with a sensitivity of 0.7 parts per million (signal-to-noise ratio of 3) over a two-week period in a bioreactor being developed at the NASA Johnson Space Center for water treatment technologies to support long-duration space missions. The feasibility of simultaneous, real-time measurements of NH(3) and CO(2) concentrations is also reported.

High-power continuous-wave mid-infrared radiation generated by difference frequency mixing of diode-laser-seeded fiber amplifiers and its application to dual-beam spectroscopy.

We report the generation of up to 0.7 mW of narrow-linewidth (<60-MHz) radiation at 3.3 micrometers by difference frequency mixing of a Nd:YAG-seeded 1.6-W Yb fiber amplifier and a 1.5-micrometers diode-laser-seeded 0.6-W Er/Yb fiber amplifier in periodically poled LiNbO3. A conversion efficiency of 0.09%/W (0.47 mWW-2 cm-1) was achieved. A room-air CH4 spectrum acquired with a compact 80-m multipass cell and a dual-beam spectroscopic configuration indicates an absorption sensitivity of +/-2.8 x 10(-5) (+/-1 sigma), corresponding to a sub-parts-in-10(9) (ppb) CH4 sensitivity (0.8 ppb).

Effective utilization of quantum-cascade distributed-feedback lasers in absorption spectroscopy.

A variable duty cycle quasi-cw frequency scanning technique was applied to reduce thermal effects resulting from the high heat dissipation of type I quantum-cascade lasers. This technique was combined with a 100-m path-length multipass cell and a zero-air background-subtraction technique to enhance detection sensitivity to a parts-in-10(9) (ppb) concentration level for spectroscopic trace-gas detection of CH4, N2O, H2O, and C2H5OH in ambient air at 7.9 micrometers. A new technique for analysis of dense high resolution absorption spectra was applied to detection of ethanol in ambient air, yielding a 125-ppb detection limit.

Methane concentration and isotopic composition measurements with a mid-infrared quantum-cascade laser.

A quantum-cascade laser operating at a wavelength of 8.1 micrometers was used for high-sensitivity absorption spectroscopy of methane (CH4). The laser frequency was continuously scanned with current over more than 3 cm-1, and absorption spectra of the CH4 nu 4 P branch were recorded. The measured laser linewidth was 50 MHz. A CH4 concentration of 15.6 parts in 10(6) ( ppm) in 50 Torr of air was measured in a 43-cm path length with +/- 0.5-ppm accuracy when the signal was averaged over 400 scans. The minimum detectable absorption in such direct absorption measurements is estimated to be 1.1 x 10(-4). The content of 13CH4 and CH3D species in a CH4 sample was determined.