Methane detection with a narrow-band source at 3.4 µm based on a Nd:YAG pump laser and a combination of stimulated Raman scattering and difference frequency mixing

We report the characterization of a 10-Hz pulsed, narrow-band source that is coincident with a fundamental ν(3) rovibrational absorption of methane at 3.43 μm. To generate this midinfrared wavelength, an injection-seeded 1.06-μm Nd:YAG laser is difference frequency mixed with first Stokes light generated in a high-pressure methane cell (1.06 ? 1.54 μm) to result in light at a wavelength of 3.43 μm, that is, the ν(1) Raman active frequency of methane (~2916.2 cm(-1)). With a modest-energy Nd:YAG laser (200 mJ), a few millijoules of this midinfrared energy can be generated with a pulse width of ~7 ns (FWHM). The methane ν(1) frequency can be pressure tuned over 8-32 atm (corresponding to ~13 GHz) and scanned across part of the ν(3)P(10) rovibrational level of methane, resulting in a peak measured methane absorption coefficient of 4.2 cm(-1) atm(-1).     

Broadly Tunable, Mode-Hop-Tuned cw Optical Parametric Oscillator Based on Periodically Poled Lithium Niobate

We describe a broadly tunable, cw optical parametric oscillator (OPO) based on periodically poled lithium niobate. The OPO can be tuned over a broad region in the mid IR (2900-3100 cm(-1)) covering the important C-H stretch region while a high spectral resolution (<0.1 cm(-1)) is maintained. The OPO is the light source for a field-portable photoacoustic spectrometer for gas-phase monitoring of volatile organic compounds.     

Doppler width limited near-infrared Raman spectrometer

A new ultra high resolution spontaneous Raman spectrometer with a single mode tunable laser diode as an excitation source and a 0.275 m, f/#4 spectrograph is presented. The spectrometer is expanded by an atomic vapor (Rb) absorption filter. One of the rubidium resonance lines serves as the frequency marker by absorbing selected Raman lines during the frequency scan of the excitation laser. High resolution was achieved while preserving the speed of the spectrometer. The extended spectrometer's capability was tested by differentiating overlapping Raman lines from molecular hydrogen isotopomers: H2, D2, and HD. Experiments were carried out at 300 K and pressures near 10(4) Pa. Spectral lines separated by approximately 10 Ghz (0.3 cm-1) can be resolved with this instrument with data collection times of minutes.     

Rotational vibrational-rotational Raman differential absorption lidar for atmospheric ozone measurements: methodology and experiment

A single-laser Raman differential absorption lidar (DIAL) for ozone measurements in clouds is proposed. An injection-locked XeCl excimer laser serves as the radiation source. The ozone molecule number density is calculated from the differential absorption of the anti-Stokes rotational Raman return signals from molecular nitrogen and oxygen as the on-resonance wavelength and the vibrational-rotational Raman backscattering from molecular nitrogen or oxygen as the off-resonance wavelength. Model calculations show that the main advantage of the new rotational vibrational-rotational (RVR) Raman DIAL over conventional Raman DIAL is a 70-85% reduction in the wavelength-dependent effects of cloud-particle scattering on the measured ozone concentration; furthermore the complexity of the apparatus is reduced substantially. We describe a RVR Raman DIAL setup that uses a narrow-band interference-filter polychromator as the lidar receiver. Single-laser ozone measurements in the troposphere and lower stratosphere are presented, and it is shown that on further improvement of the receiver performance, ozone measurements in clouds are attainable with the filter-polychromator approach.     

Ion mobility spectrometry of gas-phase ions from laser ablation of solids in air at ambient pressure

A mobility spectrometer was used to characterize gas-phase ions produced from laser ablation of solids in air at 100 degrees C and at ambient pressure with a beam focused to a diameter of 相似文献   

Fourier-transform instrument for high-resolution Raman spectroscopy of gases

An instrument is described for recording vibrational-rotational Raman spectra of gases with a resolution of 0.02-0.03 cm(-1). The Raman scattered light is collected by near forward scattering within the cavity of a single-mode, long-term, stabilized Ar-ion laser. The Raman light is analyzed in an ordinary step-scanned Michelson interferometer. To compensate for the low intensity of vibrational-rotational Raman spectra, the interferometer has a beam diameter of 160 mm. The movable mirror, weighing 2.7 kg, is mounted on a smoothly moving sledge, the stepwise motion being performed by three piezotranslators and controlled by three independent He-Ne laser beams. It is shown experimentally that it is possible for one to move the mirror with sufficient precision, using only 13% of the scan time in a typical experiment. In a preliminary spectrum of the fundamental vibration of(14) N(2), the width of the lines has been measured to 0.015 cm(-1).     

Generation of multifrequency laser emission quasi equally spaced throughout the entire visible region

A circularly polarized, monochromatic laser beam is focused into a Raman cell, which contains hydrogen to generate rotational stimulated Raman emission. After linear polarization, this two-color (separated by 587 cm(-1)) laser beam is focused several times into a second Raman cell that is filled with hydrogen to generate a multifrequency laser emission. Many rotational and vibrational lines are generated efficiently by this multipass effect. Eighteen colors that are quasi equally spaced with a rather flat intensity distribution are generated throughout the entire visible region. The present multifrequency laser emission may be advantageously used for illumination in a higher-grade display, such as a laser light show.     

Collision-activated infrared multiphoton dissociation in a quadrupole ion trap mass spectrometer

We propose and demonstrate a new method for multiple-stage mass spectrometry (MSn), collision-activated infrared multiphoton dissociation (CA-IRMPD), which is very effective for the quadrupole ion trap mass spectrometer (QITMS). CA-IRMPD uses a combination of focused laser irradiation (beam radius, approximately 0.4 mm) and collisional activation by a supplemental AC voltage between endcap electrodes. This combination enables IRMPD, which has conventionaLly been ineffective above 10(-4) Torr, to be used under a standard bath gas pressure of 2-8 mTorr. CA-IRMPD can produce richer spectra of product ions than CID or IRMPD while maintaining high sensitivity and mass resolution; thus, it will contribute to an accurate determination of peptide sequences.     

Solid hydrogen Raman shifter for the mid-infrared range (4.4-8 microm)

We developed a pulsed, continuously tunable laboratory laser source for the mid-infrared spectral range of 4.4-8 microm, which is characterized by the spectral linewidth of 0.4 cm(-1). The device is based on the stimulated backward Raman scattering in solid para-hydrogen at T = 4 K. It is pumped by a focused beam obtained from a commercial near-infrared optical parametric oscillator with output energy of approximately 20 mJ (7-ns pulse). Output energies range from 1.7 mJ at 4.4 microm to 120 microJ at 8 microm, which correspond to quantum efficiencies of 0.53 and 0.08, respectively. Spectra of NO, H2O, and CH4 molecules in the mid-infrared were recorded. The operation of the Raman cell pumped with 532-nm radiation was also studied.     

Deep-ultraviolet Raman microspectroscopy: characterization of wide-gap semiconductors

We have developed a high-throughput deep-ultraviolet (DUV) Raman microspectrometer with excitation from a continuous wave (cw) laser operated at 244 nm that enables us to characterize thin surface layers of wide-gap semiconductors. This spectrometer system consists of a filter spectrometer for the rejection of stray light and a high-dispersion spectrograph combined with a liquid nitrogen cooled charge-coupled device (CCD) detector and extends the low-frequency limit of the observable spectral range down to 170 cm(-1). In the microscope we use a Cassegrain reflective objective for the collection of the scattered light and an off-axis mirror for introduction of the excitation laser light. DUV Raman spectroscopy has been applied for studying wide-gap semiconductors including SiC and AlGaN epitaxial films and shallow implanted layers of these materials. Raman spectra of various crystals have also been measured for examining the performance of this system. Resonance enhancement of Raman bands has been observed for several semiconductors, and the results are discussed.     

Steady-state and transient ultraviolet resonance Raman spectrometer for the 193-270 nm spectral region

We describe a state-of-the-art tunable ultraviolet (UV) Raman spectrometer for the 193-270 nm spectral region. This instrument allows for steady-state and transient UV Raman measurements. We utilize a 5 kHz Ti-sapphire continuously tunable laser (approximately 20 ns pulse width) between 193 nm and 240 nm for steady-state measurements. For transient Raman measurements we utilize one Coherent Infinity YAG laser to generate nanosecond infrared (IR) pump laser pulses to generate a temperature jump (T-jump) and a second Coherent Infinity YAG laser that is frequency tripled and Raman shifted into the deep UV (204 nm) for transient UV Raman excitation. Numerous other UV excitation frequencies can be utilized for selective excitation of chromophoric groups for transient Raman measurements. We constructed a subtractive dispersion double monochromator to minimize stray light. We utilize a new charge-coupled device (CCD) camera that responds efficiently to UV light, as opposed to the previous CCD and photodiode detectors, which required intensifiers for detecting UV light. For the T-jump measurements we use a second camera to simultaneously acquire the Raman spectra of the water stretching bands (2500-4000 cm(-1)) whose band-shape and frequency report the sample temperature.     

Development of a broadband picosecond infrared spectrometer and its incorporation into an existing ultrafast time-resolved resonance Raman, UV/visible, and fluorescence spectroscopic apparatus

We have constructed a broadband ultrafast time-resolved infrared (TRIR) spectrometer and incorporated it into our existing time-resolved spectroscopy apparatus, thus creating a single instrument capable of performing the complementary techniques of femto-/picosecond time-resolved resonance Raman (TR3), fluorescence, and UV/visible/infrared transient absorption spectroscopy. The TRIR spectrometer employs broadband (150 fs, approximately 150 cm(-1) FWHM) mid-infrared probe and reference pulses (generated by difference frequency mixing of near-infrared pulses in type I AgGaS2), which are dispersed over two 64-element linear infrared array detectors (HgCdTe). These are coupled via custom-built data acquisition electronics to a personal computer for data processing. This data acquisition system performs signal handling on a shot-by-shot basis at the 1 kHz repetition rate of the pulsed laser system. The combination of real-time signal processing and the ability to normalize each probe and reference pulse has enabled us to achieve a high sensitivity on the order of deltaOD approximately 10(-4) - 10(-5) with 1 min of acquisition time. We present preliminary picosecond TRIR studies using this spectrometer and also demonstrate how a combination of TRIR and TR3 spectroscopy can provide key information for the full elucidation of a photochemical process.     

Factors determining the stability, resolution, and precision of a conventional Raman spectrometer

We verified the performance of a conventional Raman spectrometer, which is composed of a 30 cm single polychromator, a Si based charge-coupled device (CCD) camera, and a holographic supernotch filter. For that purpose, the time change of the peak positions of Raman spectra of naphthalene and fluorescence spectra of ruby (Cr-doped Al(2)O(3)) were monitored continually. A time-dependent deviation composed of two components was observed: a monotonous drift up to 0.4 cm(-1) and a periodic oscillation with a range of 0.15 cm(-1). The former component was stabilized at approximately 2000 s after the CCD detector was cooled, indicating that incomplete refrigeration of the CCD detector induced the drift. The latter component synchronized with the periodic oscillation of the room temperature, indicating that thermal expansion or contraction of the whole apparatus induced this oscillation. The implemental deviation is reduced when measurements are conducted using a sufficiently cooled CCD detector at a constant room temperature. Moreover, the effect of the room temperature oscillation is lowered in a spectrum acquired over a duration that is longer than one cycle of this oscillation. Applying the least squares fitting method to carefully measured spectra enhanced the precision of the determination of the peak position to 0.05 cm(-1) using the spectrometer with pixel resolution of 1.5 cm(-1).     

Stray Light Rejection in Rotational Coherent Anti-Stokes Raman Spectroscopy by use of a Sodium-Seeded Flame

A common experimental problem with rotational coherent anti-Stokes Raman spectroscopy (CARS) is undesired spectral interference that is due to stray light from the primary laser beams. Also, for the most developed approach, dual-broadband rotational CARS, practical measurements often suffer from stray light interference from the narrow-band laser, inasmuch as the CARS signal is produced inherently in the spectral vicinity of the narrow-band laser beam. An optical filter does not provide a sufficiently sharp transmission profile, thus leading to signal loss and spectral distortion of the rotational CARS signal. An atomic filter consisting of a sodium-seeded flame is presented here as a solution to the problem, and its usefulness was demonstrated in dual-broadband rotational CARS experiments.     

Measurement of ion beam profiles in a superconducting linac with a laser wire

A laser wire ion beam profile monitor system has been developed at the Spallation Neutron Source accelerator complex. The laser wire system uses a single laser source to measure the horizontal and vertical profiles of a pulsed hydrogen ion (H(-)) beam along a 230 m long superconducting linac, which accelerates H(-) from 200 MeV to 1 GeV. In this paper, we describe the laser optics requirement for the system, the performance of the profile measurement, and the effects of laser parameters on the measurement reliability. The result provides a practical guideline for the development of a large-scale, operational, laser-based diagnostics in accelerator facilities.     

Fiber-optic probes for in vivo Raman spectroscopy in the high-wavenumber region

In vivo Raman spectroscopy, using fiber-optic probes is hindered by the intense background signal, which is generated in the fused-silica fibers, in the fingerprint region of the Raman spectrum (approximately 0-2000 cm(-1)). Optical filtering is necessary to obtain tissue spectra of sufficient quality. The complexity of fiber-optic probes for fingerprint Raman spectroscopy, in combination with size constraints and flexibility requirements for in vivo use have been a major obstacle in the development of in vivo diagnostic tools based on Raman spectroscopy. A setup for remote Raman spectroscopic tissue characterization in the high-wavenumber region ( approximately 2400-3800 cm(-1)) is presented. It makes use of a single, unfiltered, optical fiber for guiding laser light to the sample and for collecting scattered light and guiding it back to a spectrometer. Such a simple configuration is possible because the fused-silica core and cladding of the fiber present almost no Raman background signal at these wavenumbers. Several commercially available optical fibers were tested with respect to Raman signal background, to determine their suitability for in vivo Raman spectroscopy measurements in the high-wavenumber region. Different fiber core, cladding, and coating materials were tested. Silica core-silica clad fibers, with an acrylate coating and a black nylon jacket, proved to be one of the best candidates. In vitro measurements on brain tissue of a 6-month-old pig were obtained with a remote high-wavenumber Raman setup. They illustrate the low background signal generated in the setup and the signal quality obtained with a collection time of 1 s.     

Ion-molecule reactions in a tandem mass spectrometer as a source of state selected ions for high energy collision spectroscopy

This paper suggests that a tandem mass spectrometer of perpendicular type and a high energy collision spectrometer united in a single instrument can be used for studying the properties of state selected ions by making use of the traditional methods of high energy collision spectroscopy. The tandem mass spectrometer serves to produce a continuous or pulsed beam of mass and state selected ions via low energy ion-molecule reactions in the gas phase. The high energy collision spectrometer is then used to investigate the interactions of these state selected ions with a gas target, a solid foil or a solid surface. Principal applications of the suggested new combination of instruments may include Coulomb explosion spectroscopy and beam-foil spectroscopy as well as surface analysis by using state selected ions. Excitation of the beam of state selected ions with a laser beam or synchrotron radiation is also possible.     

Compact 50-Hz terawatt Ti:sapphire laser for x-ray and nonlinear optical spectroscopy

We report a high-repetition-rate, compact terawatt Ti:sapphire laser system. The oscillator produces an 82-MHz pulse train consisting of broad-bandwidth pulses of 0.5-nJ/pulse energy and of 9-fs pulse duration. The spectrally shaped, lambda/4 regenerative amplifier supports an 80-nm bandwidth. A single 50-Hz repetition-rate pump laser pumps both the regenerative amplifier and a multiple-pass amplifier. The final output from this laser is a 50-Hz pulse train made from pulses of 53 mJ/pulse energy and of 24-fs pulse duration. For generating ultrafast x-ray pulses, 90% of the energy from the final output of a 28-mm-diameter (1/e2) beam is focused onto an ultrafast x-ray wire target. The energy conversion efficiency from optical (800-nm central wavelength) to x-ray (characteristic lines of K(alpha) from Cu at 8 keV) pulses is estimated to be 7 x 10(-5). This laser system can also generate a lower-peak-power, dual-pulse output that can excite, simultaneously and coherently, Raman modes within an adjustable bandwidth (up to 700 cm(-1)) and at a tunable central vibrational frequency. Preliminary results for the generation of dual-pulse output and ultrafast x rays are presented.     

A developed optical-feedback cavity ring-down spectrometer and its application

A developed spectrometer based on optical-feedback cavity ring-down spectroscopy (OF-CRDS) has been demonstrated with a distributed feedback laser diode and a V-shaped glass ceramic cavity. The laser is coupled to the V-shaped cavity, which creates an absorption path length greater than 2.8 km, and resonance between the laser frequency and the cavity modes is realized by modulating the cavity length instead of tuning the laser wavelength to obtain a higher resolution. A noise-equivalent absorption coefficient of ~2.6 × 10(-8) cm(-1)Hz(-1/2) (1σ) is determined with spectral resolution of ~0.003 cm(-1) and spectral range of 1.2 cm(-1). As an application example, the absorption spectrum measurement of water vapor in the spectral range of 6590.3~6591.5 cm(-1) is demonstrated with this spectrometer.     

Determination of spectrometer-detector parameters from calibration spectra and the use of the parameters in spectrometer calibrations

The grating equation is used to generate quadratic calibration equations for multichannel detectors with perpendicular and tilted focal planes. The quadratic coefficients are not independent and contain terms that are used to solve for spectrometer-detector parameters. The parameters can be calculated from a quadratic fit at one spectrometer position, but more accurate values can be obtained from quadratic fits at two spectrometer positions. The calculations show that the detector focal plane is tilted by about two degrees. Once values for the spectrometer-detector parameters are obtained from calibrations using at least three lines at one or two spectrometer positions, only one calibration line at any spectrometer position is required to obtain accuracies on the order of 0.1 cm(-1) over a several thousand wavenumber range. The main cause of spectrometer drift is a change in the diffraction angle and/or the spectrometer included angle. This drift is almost totally compensated by the one-line calibration, which adjusts the diffraction angle. A neon pen lamp is used to generate the calibration spectra. Using standard air wavelengths compared to true wavelengths can produce calibration errors of 0.1 to 0.6 cm(-1); the magnitude depends on local conditions and how the laser wavelength is treated.