Doppler-broadened Absorption Line Research Articles

Improving pulsed laser induced fluorescence distribution function analysis through matched filter signal processing.

Laser induced fluorescence is used to measure argon ion heating during magnetic reconnection in the PHase Space MApping experiment (PHASMA). Sufficient signal-to-noise ratio (SNR) of the processed signal with pulsed laser injection is a delicate balance between saturation of the absorption line and injecting enough laser power to overcome the spontaneous emission of the plasma at the fluorescence wavelength. Averaging over many laser pulses and integrating over the fluorescence lifetime improves the SNR of the processed signal (processed SNR) when the SNR of the laser pulse time series is small (pulse SNR), but for laser powers small enough to avoid saturation, averaging over hundreds of pulses is needed to obtain an appreciable processed SNR over the entire Doppler-broadened absorption line. Here, we describe a matched filter processing method that significantly improves the SNR of the final measurement with fewer shots averaged. Investigation of simulated measurements validated by experimental results suggests that the matched filter method provides up to a 20% improvement in the processed SNR, resulting in less uncertainty in distribution function fits.

The classical Doppler-broadened absorption line profile due to thermal effect

The classical Doppler-broadened absorption line profile, which is exactly derived by the classical Doppler effect and Maxwell speed distribution for the first time, is asymmetrically broadened with red-shift more than blue-shift, and its peak is red-shifted. Nonetheless, the half-width at half-maximum in the blue-shift wing is larger than that in the red-shift wing. The classical Doppler-broadened absorption line profile is different from the classical Doppler-broadened emission line profile. The classical Doppler-broadened absorption line profile is applied to disprove one of the two existing relativistic Doppler-broadened absorption line profiles. Also, line-shape models based on the classical Doppler-broadened absorption line profile are suggested for absorption line spectroscopy.

Self-referencing mW-scale detection of sub-ns optical phase modulation in acetylene at 1530 nm.

Application of the phase memory of acetylene (C2H2) vibrational-rotational transitions in the 1520-1540nm wavelength range for a self-referencing homodyne detection of a sub-nanosecond optical phase modulation is reported. In the proposed configuration the collinearly propagating coherent dipole radiation of the excited molecule acts like a phase-locked reference wave (local oscillator) that enables transformation of the initial phase modulation into the intensity one. This technique does not need high light intensity and can operate in a linear range of the medium optical absorption. The linear optical phase demodulation (i.e., transformation of the phase-to-amplitude light modulation) is interpreted as an introduction of an additional phase shift to the carrier frequency component of the modulated signal in the maximum of the dispersion curve of a narrow optical absorption peak. It has been experimentally demonstrated with the bulk 10cm long cuvette filled with low pressure (∼2 Torr) acetylene at room temperature. Effective demodulation of the milliwatt-scale incident laser wave of a single- and multi-mode structure is shown. As expected, the response to the fast (<1 ns) phase modulation was quadratic when the acetylene inhomogeneous Doppler-broadened (∼500 MHz) absorption line is excited in its center and was linearized by tuning at one of the absorption line sides. It is of a differentiating (high-pass) type with the cutoff frequency determined by the total spectral width of the utilized absorption line. Expected detection resolution is determined by the photon noise of the incident light intensity.

Sinusoidal frequency modulation on laser diode for frequency stabilization and displacement measurement

Abstract In this paper, we propose the sinusoidal frequency modulation on a laser diode (LD) to achieve both frequency stabilization of the LD and displacement measurement with a homodyne interferometer. The central frequency of the LD is stabilized to a Doppler broadened absorption line of iodine (I 2 ) molecule near the wavelength of 633 nm. The frequency of the LD is modulated across the absorption line and synchronous detection is utilized to detect the absorption signal. A simple and low-cost homodyne displacement-measuring interferometer can be constructed that can attain high measurement resolution.

Accurate determination of the Boltzmann constant by Doppler spectroscopy: Towards a new definition of the kelvin

Accurate molecular spectroscopy in the mid-infrared region allows precision measurements of fundamental constants. For instance, measuring the linewidth of an isolated Doppler-broadened absorption line of ammonia around 10 $\mu$m enables a determination of the Boltzmann constant k B. We report on our latest measurements. By fitting this lineshape to several models which include Dicke narrowing or speed-dependent collisional effects, we find that a determination of k B with an uncertainty of a few ppm is reachable. This is comparable to the best current uncertainty obtained using acoustic methods and would make a significant contribution to any new value of k B determined by the CODATA. Furthermore, having multiple independent measurements at these accuracies opens the possibility of defining the kelvin by fixing k B, an exciting prospect considering the upcoming redefinition of the International System of Units.

Cavity ring-down spectroscopy of Doppler-broadened absorption line with sub-MHz absolute frequency accuracy

A continuous-wave cavity ring-down spectrometer has been built for precise determination of absolute frequencies of Doppler-broadened absorption lines. Using a thermo-stabilized Fabry-Pérot interferometer and Rb frequency references at the 780 nm and 795 nm, 0.1 - 0.6 MHz absolute frequency accuracy has been achieved in the 775-800 nm region. A water absorption line at 12579 cm(-1) is studied to test the performance of the spectrometer. The line position at zero-pressure limit is determined with an uncertainty of 0.3 MHz (relative accuracy of 0.8 × 10(-9)).

Measurement of the absorption line profiles of water vapour isotopomers at 1.39 μm using the methods of diode laser spectroscopy

The issues related to high-precision measurement of the absorption line profiles of water vapour and its isotopomers using the methods of diode laser spectroscopy in the near IR range aimed at the analysis and detection of greenhouse gases are considered. The absorption line shape of H216O is investigated as a function of pressure of different buffer gases. The influence of the instrument function of the diode laser (DL) on the precision of measuring the line profile is studied. From fitting the profile of Doppler-broadened H216O absorption line to a model profile the lasing line width of the DL with a fibre pigtail is determined. The frequencies and intensities of absorption lines of water isotopomers H216O, H217O, H218O, and HDO are measured in the range of DL oscillation. Analytical spectral regions are chosen for distant probing of water vapour using an airborne lab.

Infrared laser-based monitoring of the silane dissociation during deposition of silicon thin films

The silane dissociation efficiency, or depletion fraction, is an important plasma parameter by means of which the film growth rate and the amorphous-to-microcrystalline silicon transition regime can be monitored in situ. In this letter we implement a homebuilt quantum cascade laser-based absorption spectrometer to measure the silane dissociation efficiency in an industrial plasma-enhanced chemical vapor deposition system. This infrared laser-based diagnostic technique is compact, sensitive, and nonintrusive. Its resolution is good enough to resolve Doppler-broadened rotovibrational absorption lines of silane. The latter feature various absorption strengths, thereby enabling depletion measurements over a wide range of process conditions.

Theory of Dicke narrowing in coherent population trapping

The Doppler effect is one of the dominant broadening mechanisms in thermal vapor spectroscopy. For two-photon transitions one would naively expect the Doppler effect to cause a residual broadening, proportional to the wave-vector difference. In coherent population trapping (CPT), which is a two-photon narrow-band phenomenon, such broadening was not observed experimentally. This has been commonly attributed to frequent velocity-changing collisions, known to narrow Doppler-broadened one-photon absorption lines (Dicke narrowing). Here we show theoretically that such a narrowing mechanism indeed exists for CPT resonances. The narrowing factor is the ratio between the atom's mean free path and the wavelength associated with the wave-vector difference of the two radiation fields. A possible experiment to verify the theory is suggested.

Rapid passage induced population transfer and coherences in the 8 micron spectrum of nitrous oxide

Rapid passage signals showing the effects of molecular alignment have been observed when low pressure samples of nitrous oxide are interrogated by radiation from a pulsed 7.84 µm quantum cascade laser. These effects occur when the sweep rate of the laser through a Doppler broadened absorption line is much faster than the collisional relaxation time, and when the power density of the linearly polarized laser radiation is sufficient to cause optical pumping. Using a laser pulse of duration 1.3 µs, the frequency sweeps approximately 90 GHz. The variation of the laser tuning rate during the laser pulse, from about100 MHz/ns at the beginning to about 20 MHz/ns at the end, allows the relationship between sweep rate and collisional damping to be investigated. It is shown, by comparing the experimental signals with those calculated by coupled Maxwell–Bloch equations, how the rapid passage effects in nitrous oxide are influenced by the number density, transition cross-section and reorientation lifetime.

An experiment to measure the electron–ion thermal equilibration rate in a strongly coupled plasma

We present the most recent results from an experiment aimed at obtaining the temperature equilibration rate between ions and electrons in a strongly coupled plasma by directly measuring the temperature of each component. The plasma is formed by heating a sonic gas jet with a 10 ps laser pulse. The electrons are preferentially heated by the short pulse laser (we are aiming for Te ~ 100 eV), while the ions, after undergoing very rapid (sub-ps timescale) disorder-induced heating, should only reach a temperature of 10–15 eV. This results in a strongly coupled ion plasma with a Γii ~ 3–5. We plan to measure the electron and ion temperatures of the resulting plasma independently during and after heating, using collective Thomson scattering for electrons and a high-resolution x-ray spectrometer for the ions (measuring Doppler-broadened absorption lines). Theory indicates that the equilibration rate could be significantly lower than that given by the usual weakly coupled model (Landau–Spitzer) due to coupled collective modes present in the dense plasma.

Observation of saturation and rapid passage signals in the 10.25 micron spectrum of ethylene using a frequency chirped quantum cascade laser

Rapid passage signals exhibiting saturation effects have been observed when a low-pressure sample of ethylene, within a multiple pass absorption cell, is subjected to radiation from a repetitively pulsed 10.25 micron quantum-cascade laser. Within each pulse the laser frequency sweeps 36 GHz from high to low frequency in a time of 140 ns. At the low gas pressures, less than 20 mTorr, in the absorption cell the sweep rate through a Doppler-broadened absorption line (ca. 0.5 ns), is much faster than the collisional relaxation time of the ethylene and this leads to rapid passage effects. Examples are given of the complex rapid passage signals observed in pure and nitrogen broadened spectra. The rapid passage effects, which lead to the variety of the observed signals, have been modelled by numerical solution of the coupled Maxwell–Bloch equations for four sets of two-level systems.

Molecular Transitions as Medium-Precision Wavelength Standards for Optical Communication

Molecular vibration-rotation transitions in overtone or combination bands are well suited as absolute wavelength standards throughout the wavelength range used for optical communication. In this paper, we describe two different implementations. In the first, radiation from a superluminescent diode is sent through a cell containing H/sup 13/CN. This provides the spectrum with a characteristic signature from absorption in the 2/spl nu//sub 3/ overtone band, and allows for calibration in the 1530-1560 nm range. In the second implementation, radiation from a distributed feedback (DFB) fiber laser is locked to a Doppler broadened absorption line at 1548.8193 nm in the /spl nu//sub 1/+/spl nu//sub 3/ combination band of /sup 13/C/sub 2/H/sub 2/. The resulting standard has a relative Allan deviation of less than 10/sup -8/ beyond 100 s of integration time, and the signal can be disseminated through optical networks without degradation in stability. A novel feature of the fiber laser is all thermal control of the wavelength through resistive heating of a thin-film gold coating.

Submillimeter-wavelength plasma chemical diagnostics for semiconductor manufacturing

Submillimeter-wavelength linear-absorption spectroscopy has been applied to the chemical diagnostics of reactive-ion etching plasmas in a modified capacitively coupled gaseous electronics conference reactor. Approximately 1 mW of narrow-band (&amp;lt;10 kHz) submillimeter radiation between 450 and 750 GHz is produced using a backward-wave oscillator (BWO). The BWO is frequency stabilized to a harmonic of a 78–118 GHz frequency synthesizer. The submillimeter method offers high sensitivity for the ≈1 MHz linewidth, Doppler-broadened absorption lines typical of gas-phase molecules at a total pressure of less than 133 Pa (1 Torr). A large number of molecules can be detected, limited primarily by the need for a permanent electric dipole moment and for accurate line frequency predictions, the latter of which are often available in the literature. The capabilities of the diagnostic method have been demonstrated by the following three applications: (1) the measurement of water-vapor contamination in the reactor and in the precursor gas by monitoring a rotational transition of H2O in the reactor just prior to the initiation of the plasma; (2) the assessment of progress in the cleaning of the reactor by an O2/Ar plasma after a fluorocarbon plasma etch by monitoring the build up of the concentration of O3 and the depletion of the concentration of CF2O in the plasma; and (3) the determination of the endpoint in the etching of a SiO2 thin film on silicon by an octafluorocyclobutane/O2/Ar plasma by monitoring the decrease in the concentration of SiO in the plasma. The last observation is made possible by the large electric dipole moment for SiO of 1×10−29 C m (3.1 D), which gives a low minimum detectable number density for the radical of 2×107 cm−3 for an optical pathlength of 39 cm.

In-line laser Doppler velocimeter using fibre-optic Bragg grating interferometric filters

An all fibre-based laser Doppler velocimetry system for the measurement of thepredominantly on-axis component of velocity is presented. The technique employsBragg grating-based Fabry–Perot interferometric filters custom-designed to suitthe transduction of Doppler optical frequency shifts to intensity. The sensitivity ofthe in-fibre filters to strain is exploited to provide tuning of the filter relative tothe centre wavelength of the laser and of the spectral shape of the filter, which inturn varies the velocity range, sensitivity and resolution. A phase-locking schemeincorporated to stabilize the filter is described together with a laser-wavelengthstabilization system that locks to a Doppler-broadened absorption line of iodinevapour. An optical fibre-linked probe head is used to couple the laser beamat 514.5 nm to and from the measurement volume. The velocimeter ischaracterized by measuring the velocity of a rotating disc over ranges of ±42and ±7 m s−1with a resolution of 0.2 m s−1.

Wavelength references for 1300-nm wavelength-division multiplexing

We have conducted a study of potential wavelength calibration references for use as both moderate-accuracy transfer standards and high-accuracy National Institute of Standards and Technology (NIST) internal references in the 1280-1320-nm wavelength-division-multiplexing region. We found that most atomic and molecular absorption lines in this region are not ideal for use as wavelength references owing to factors such as weak absorption, complex spectra, or special requirements (for example, frequency-doubling or excitation with an additional light or discharge source). We have demonstrated one of the simpler schemes consisting of a tunable diode laser stabilized to a Doppler-broadened methane absorption line. By conducting a beat-note comparison of this reference to a calcium-based optical frequency standard, we measured the methane line center with an expanded uncertainty (2/spl sigma/) of /spl plusmn/2.3 MHz. This methane-stabilized laser now serves as a NIST internal reference.

Methylation effects in state resolved quenching of highly vibrationally excited azabenzenes (Evib∼38 500 cm−1). I. Collisions with water

To investigate the role of molecular structure in collisions that quench highly vibrationally excited molecules, we have performed state resolved transient infrared absorption studies of energy gain in a number of rotational levels of H2O(000) resulting from collisions of water with vibrationally excited 2-methylpyridine (2-picoline) and 2,6-dimethylpyridine (2,6-lutidine) in a low-pressure gas-phase environment at 298 K. Vibrationally excited methylpyridines were prepared with ∼38 500 cm−1 of internal energy using 266 nm ultraviolet excitation to an S1 electronic state followed by rapid radiationless decay to the S0 electronic state. Collisions that populate rotationally excited states of H2O(000) were investigated with infrared absorption by monitoring the appearance of individual rotational states of H2O(000) with energies between 1000 and 2000 cm−1. Rotational state distributions for recoiling water molecules were characterized by Boltzmann temperatures of Trot=590±90 K for quenching of hot picoline and Trot=490±80 K for lutidine quenching. Doppler-broadened transient absorption line profiles show that the scattered H2O(000) molecules have laboratory-frame translational energy distributions corresponding to Ttrans≈600 K for deactivation of picoline and Ttrans≈590 K for lutidine. Energy transfer rate constant measurements indicate that rotational excitation of H2O(000) with Evib&amp;gt;1000 cm−1 occurs for one in 31 picoline/water collisions and one in 17 lutidine/water collisions. Comparison with earlier quenching studies on pyrazine [M. Fraelich, M. S. Elioff, and A. S. Mullin, J. Phys. Chem. 102, 9761 (1998)] and pyridine [M. S. Elioff, M. Fraelich, R. L. Sansom, and A. S. Mullin, J. Chem. Phys. 111, 3517 (1999)] indicate that, for the same initial internal energy in the hot donor, the extent of rotational excitation in water is diminished as the number of vibrational modes in the donor increases. The energy transfer probability for this pathway exhibits opposite behavior, with the larger donor molecules being more likely to excite the high energy rotations in water. These results are interpreted using a statistical description of the high energy donors and highlight the role of low frequency vibrational modes in the vibrationally hot donor molecules. A Fermi’s golden rule approach is successful at explaining differences in the observed scattering dynamics for the various donor molecules.

Kinetics of Atomic Nitrogen Photofragment Produced by Laser Photodissociation of N2O

The kinetics of ground-state atomic nitrogen photofragments produced by laser photodissociation of nitrous oxide have been investigated using two-photon LIF. A single 207 nm laser pulse was used for both N2O photolysis and N atom two-photon LIF. The dependency of the LIF signal with laser power indicated that the observed N atom fragment was produced by N2O dissociation via single-photon absorption. Effects of translational energy of the N atom fragment were detected in collisional quenching rates of the two-photon excited N atom (3p)4S state as observed in the decay lifetime of the induced fluorescence. The mean translational kinetic energy of the N atom fragment was determined to be 0.6 ± 0.2 eV from the quenching data. An analysis of the Doppler broadened absorption line shape of the recoiling atomic nitrogen confirmed the mean kinetic energy and further presented a model speed distribution and anisotropy parameter that was consistent with the line shape data. The NO translational and internal energies...

Role of noise in the diode-laser spectroscopy of the spectral line profile

Questions concerning precise measurements of the spectral-line-profile parameters by diode-laser spectroscopic methods were examined. The instrumental function of a distributed-feedback diode laser (λ =1.53 μm), consisting of the additive contributions of the noise due to spontaneous emission, frequency fluctuations, and intensity fluctuations, was investigated. An analytical formula was obtained for the spectrum of the diode-laser field formed by frequency fluctuations. The spectral density g0 of the frequency fluctuations, determining the width of the central part of the emission line profile of a diode laser, was found by two independent methods (by fitting to a Doppler-broadened absorption line profile and by finding the intensity of the residual radiation and the saturated-absorption line width). The parameters Ω and Γ of the spectral density of the frequency fluctuations, coupled to the relaxation oscillations and determining the wing of the diode-laser emission line profile, were determined experimentally. By taking into account the instrumental function of the diode laser, involving successive convolution with the recorded emission spectra, it was possible to reproduce correctly the spectral line profile and to solve accurately the problem of the 'optical zero'. The role of the correlation between the intensity noise and the diode-laser frequency was considered.

State-Resolved Studies of Collisional Quenching of Highly Vibrationally Excited Pyrazine by Water: The Case of the Missing V → RT Supercollision Channel

The quenching of highly vibrationally excited pyrazine through collisions with H2O at 300 K in a low-pressure environment was investigated using high-resolution transient absorption spectroscopy of water at λ ≈ 2.7 μm. Highly vibrationally excited pyrazine with Evib = 37 900 cm-1 was prepared by absorption of 266 nm light to the electronically excited S2 state, followed by rapid radiationless decay to the ground electronic state. Collisions between highly excited pyrazine and water that result in rotational and translational excitation of the vibrationless ground state of H2O (000) were investigated by measuring the state-resolved appearance of individual rotational states of H2O (000). Transient absorption measurements have been made on numerous rotational states to determine the nascent distribution of rotational energy gain in water. Doppler-broadened transient absorption line shapes were collected for a number of rotational levels in the (000) state in order to measure velocity distributions of the scattered water molecules. The nascent distribution of water rotational states with Erot > 1000 cm-1 is well described by Trot = 920 K, and the velocity distributions correspond to Ttrans ≈ 560 K, independent of the rotational state. Rate constants for energy gain into individual quantum states of H2O (000) from collisions with hot pyrazine provide a measure of the high-energy part of the energy-transfer probability distribution function. The quenching of hot pyrazine through collisions with water displays a significant reduction in the bath translational energy gain when compared to earlier studies on the quenching of hot pyrazine (Evib = 37 900 cm-1) by CO2 {Wall, M. C.; Mullin, A. S. J. Chem. Phys. 1998, 108, 9658}. A comparison of the two systems provides insights into the molecular properties that influence the relaxation of highly vibrationally excited molecules.