Intracavity Laser Spectroscopy Research Articles

Numerical investigation of the influence of a nonlinear absorber on the sensitivity of intracavity laser spectroscopy

It is shown, by numerical modeling in combination with experimental data, that the use of a nonlinear absorber as a spectral contrast enhancer can ensure a real increase in the sensitivity of the method of intracavity laser spectroscopy in the microsecond range by one or two orders of magnitude, without increasing the duration of lasing. Further optimization of all the experimental parameters (using a stronger focusing system, lowering the intrinsic resonator losses, raising the optical density of a nonlinear filter) should increase the sensitivity to weak absorption by three or four orders of magnitude relative to that obtained using normal intracavity measurements.

Ultraviolet-laser photolysis of disilane

A combination of optical probing techniques, including time-resolved emission spectroscopy, dye-laser absorption spectroscopy, laser-induced fluorescence, and intracavity laser spectroscopy (ILS), have been used to investigate the reaction paths for the ArF excimer laser (193 nm, 6.4 eV/photon) photolysis of disilane, Si2H6. The experiments were carried out in an ultrahigh vacuum system into which Si2H6 was introduced at pressures ranging from 1 to 10 mTorr. Optical emission due to the Si 1P→1S and SiH A2Δ→X2Π transitions was observed with intensity rise times, following the laser pulse, of less than 100 ns, the experimental resolution. This indicates that for the pressures used in these experiments, Si and SiH were generated purely photolytically rather than through collisional processes. The maximum intensities of the Si and SiH peaks were measured as a function of ArF laser-beam photon flux density Jhν. SiH2 was never detected, up to the highest Jhν values used, 4×1017 photons/cm2 pulse, even though with ILS, the most sensitive of the optical-absorption techniques, we were able to detect absorption peaks assignable to rotational transitions in the (020)–(000) vibronic bands of the à 1B1←X̃ 1A1 electronic transition during Si2H6 pyrolysis at ∼350 °C. Based upon these experimental results, a model for the ultraviolet-laser photolysis of Si2H6 was developed involving a three-step cascade one-photon absorption process.

Methane overtone absorption by intracavity laser spectroscopy

The line intensity and self-broadening coefficient of the 619.68 nm overtone absorption line of methane are measured at room temperature using intracavity laser spectroscopy (ILS). The methodology appropriate for quantitative analysis of absorption lineshapes with ILS is determined by data recorded for several absorption lines of the well-studied oxygen γ-band. The values for methane line intensity ( S=0.119±0.009 cm −1 atm −1 km −1) and self-broadening coefficient (γ c=0.090±0.007 cm −1 atm −1) found in this study are in agreement with previously published values obtained with other experimental techniques.

On the reaction of the NH 2 radical with NO 2 at 295–620 K

The reaction of the NH 2 radical with NO 2 (NH 2+NO 2→products) was investigated using intracavity laser spectroscopy over a broad range of temperatures (295–620 K) and pressures (10–650 Torr). The temperature dependence of the rate constant, k 1( T), over the range 295–620 K is well approximated by the expression k 1( T)=(2.1±0.4)×10 −11( T/298) −1.7 cm 3/s. The pressure experiments showed that k 1 is independent of pressure.

Combined influence of quantum fluctuations and spatial inhomogeneity of the dynamics of multimode lasers, and sensitivity of intracavity laser spectroscopy

Combined influence of quantum fluctuations and spatial inhomogeneity on the mode dynamics of multimode lasers and the sensitivity of intracavity spectroscopy has been analyzed. It has been shown that this influence might be essential for laser pulses as long as 1 ms. The sensitivity of intracavity spectroscopy saturates after the same duration of laser emission.

I n s i t u detection of BH2 and atomic boron by intracavity laser spectroscopy in the plasma dissociation of gaseous B2H6

Absorption spectra of the BH2 radical and atomic B are detected by intracavity laser spectroscopy (ILS) in situ during the plasma dissociation of gaseous diborane (B2H6). Rovibronic absorption transitions of BH2 are observed near 15 469 cm−1 [(0,11,0)2B′1 –(0,0,0)2A″1] and 13 605 cm−1 [(0,9,0)2B′1 –(0,0,0)2A″1]. Absorption corresponding to the 3p (2P0)-5s (2S) transitions of atomic B are observed at 11 534.6 and 11 532.8 cm−1. Although atomic B has been detected previously under experimental conditions used in the plasma-assisted preparation of boron-containing films from B2H6, the ILS data presented here establish, for the first time, the presence of BH2 under chemical vapor deposition conditions.

On the reaction of the NH 2 radical with SO 2 at 298–363 K

The reaction of the NH 2 radical with SO 2 at 298–363 K was investigated using the intracavity laser spectroscopy technique. The high and low pressure values of the rate constant for the reaction NH 2+SO 2+M→NH 2SO 2+M(1) at 298 K are as follows: k 1∞ = (1.5±0.3) × 10 −13 cm 3 s −1, k 1,NH 3 0 = (2.1±0.4) × 10 −31 cm 6 s −1 and k 1,N 2 0 = (3.1±0.6) × 10 −32 cm 6 s −1. The temperature dependence of k 1∞ ( T) over the range 298–363 K is well approximated by the expression k 1∞ ( T) = (1.5±0.3) × 10 −13 ( T/298) −(1.3±0.3). Experimental data together with RRKM calculations give an estimate of 25 −1 +2 kcal/mole for the NS bond energy in NH 2SO 2.

Dynamics of population of the A3∑u+nitrogen metastable state in a self-sustained volume discharge of a pulsed CO2laser

Intracavity laser spectroscopy was used to study the dynamics of population of the ν = 2–8 vibrational levels of the A3∑u+ state in order to establish the possible influence of multistage ionization on the evolution of instability in a self-sustained volume discharge in CO2 laser active mixtures. The populations of the nitrogen vibrational levels Nν were calculated taking into account the real output pulse profile of a dye laser. It was found that multistage ionization can only influence the duration of stable operation of a self-sustained volume discharge by increasing the rate of growth of the spark channel in the discharge gap. This is why the addition of readily ionized substances to the gas that reduce the electron energy and therefore lower Nν can substantially improve the stability of the volume discharge and increase the active volume and output energy of a CO2 laser.

Intracavity laser spectroscopy

The main features of broadband intracavity laser spectroscopy (ILS), the most sensitive technique of absorption-spectrum measurements, is investigated. The factors that limit the sensitivity of the ILS method are studied, and the conditions for reaching its maximum sensitivity are formulated. The main practical applications of the ILS method are described, such as detection of weak absorption spectra and measurements of low gas concentrations, registration of amplification spectra and of the spectral distribution of external radiation, measurement of the cross sections for nonlinear processes, investigation of transient processes in an electric discharge, and investigation of the kinetics of multistage chemical reactions.

Role of silylene in the pyrolysis of silane and organosilanes

The role of the silylene radical, SiH/sub 2/, in the pyrolytic decomposition of silane and organosilanes is examined. Silylene is detected directly in absorption by intracavity laser spectroscopy (ILS). This represents the first reported detection of SiH/sub 2/ during the pyrolytic decomposition of silanes. The high detection sensitivity of ILS for absorption measurements enables the in situ examination of the pyrolytic decomposition processes under reaction conditions chosen to facilitate film growth rather than to optimize spectroscopic detection. The relative amount of gas-phase SiH/sub 2/, as measured by ILS data during chemical vapor deposition (CVD), correlates well with changes in the temperature and gas-flow rates in the pyrolysis chamber and with semiquantitative observations of the rate of film growth. These results demonstrate the importance of homogeneous reactions such as those involving SiH/sub 2/ in the pyrolytic CVD of silicon films.

Intracavity laser spectroscopy with nonlinear frequency conversion

A new absorption spectroscopy method is proposed. This method is based on intracavity generation of sum and difference frequencies and it is analogous, in respect of its physics and capabilities, to intracavity laser spectroscopy. An investigation is reported of the dependence of the sensitivity of the new method on the efficiency of nonlinear frequency conversion and on the phase relationships obeyed by waves interacting in a nonlinear crystal. It is reported that the sensitivity of the new method in determination of weak absorption is between two and three orders of magnitude higher than the extracavity variant.

The A4η-X4Σ System op Tiie Mon Molecule

Abstract Absorption spectrum of MoN has been obtained in the 500–670 nra region by intracavity laser spectroscopy. Vibrational analysis of A > I Z transition has been carried out. Molecular constants for the upper (A4η) and ground (X4Σ) states have been determined.

The Electronic Absorption Spectrum of Zirconium Monooxide

Abstract The absorption spectrum of ZrO has been obtaioed io the 560-670 nm region by intracavity laser spectroscopy. Vibrational analyses of e3Π -a3Δ and d3Φ -a3Δ transitions have been carried out with accounting for an anomalous multiplet splitting. Molecular constants for the upper e3Π and d3Φ states have been determined.

ULTRA-SENSITIVE INTRACAVITY SPECTROSCOPY WITH MULTIMODE LASERS

Intracavity laser spectroscopy is characterized by extreme sensitivity of the emission spectrum to narrow spectral perturbations such as absorption, gain or light injection. Intracavity absorption spectra obey a modified Lambert-Beer law, where the length of the absorption cell is substituted by 1 = c.t, where c is the velocity of light, and t is the duration of the laser pulse. The time resolution of intracavity measurements is limited only by the sensitivity required for the detection of the extinction k, such that i ≥ 1/kc. With the minimum detectable absorption being 10-5 cm-1, e.g., the resolvable time is on the order of a microsecond. The ultimate sensitivity of intracavity spectroscopy with a cw laser is limited by one of two competing factors : spontaneous emission, and non-linear mode interaction, such as stimulated Brillouin scattering. Depending on laser parameters, minimum detectable extinction is in the range 10-7-10-12 cm-1. Non-linear mode interaction can also give rise to distortion of the line shapes observed with intracavity spectroscopy. High sensitivity along with time resolution opens a wide field of practical application for intracavity spectroscopy such as pollution detection, detection of forbidden and non-linear transitions, combustion and plasma diagnostics, and the study of kinetics of molecules and radicals.

Polariton mechanism of condensation of the emission spectrum of a laser near an absorption line

An investigation is made of the condensation of the emission spectrum of a multimode laser near a strong absorption line of an intracavity gas cell associated with a strong frequency dispersion of polaritons in a gas. It is shown that the polariton mechanism includes—in addition to the usual (in intracavity laser spectroscopy) change in the Q factor and, consequently, in the mode amplitudes—an increase in the density of polariton mode frequencies.

Intracavity laser spectroscopy: propagation of absorption data through optical fibers

The propagation of achromatic visible laser radiation through optical fibers is examined for a quasi-cw dye laser designed to record absorption by intracavity laser spectroscopy (ILS). The wavelength and intensity characteristics of the ILS laser output and any superimposed intracavity absorption features are preserved quantitatively for propagation through an extracavity single-mode optical fiber. Feedback from the front surface of the optical fiber and its associated optics can modify the ILS laser output. Two optical configurations which give no noticeable modifications to the IS data propagated through the optical fiber are described. Applications of ILS for remote sensing, sensitive detection of extracavity feedback effects, and characterization of extracavity interferometric signals are indicated.

Detection of spectrally narrow light emission by laser intra-cavity spectroscopy

The spectral distribution of the output of a multimode laser is extremely sensitive to narrow-band absorption, gain, or light injection. We demonstrate dramatic redistribution in such output spectra to depend on the time delay of the injection. This effect seems to be useful for spectrally resolved detection of weak light signals.

Method of quantitative measurements of one- and two-photon absorption spectra obtained by intracavity laser spectroscopy

Method of quantitative measurements of one- and two-photon absorption spectra obtained by intracavity laser spectroscopy

Kinetics of emission spectra of multimode lasers and its influence on the sensitivity of intracavity laser spectroscopy

Theoretical and experimental investigations were made of fluctuations of the intensity of radiation representing individual modes of a multimode laser. A model was developed for the nonlinear interaction of modes and numerical calculations were made of the kinetics of lasing allowing both for the interaction between the modes and for quantum fluctuations. It was found that the only factor governing the sensitivity of intracavity spectroscopy is the duration of lasing in each mode, which in turn may be governed either by quantum fluctuations or by the mode interactions. The optimal laser radiation power ensuring maximum sensitivity of intracavity spectroscopy was found. A satisfactory agreement was obtained beween experimental and calculated results.

Experimental and theoretical investigation of the electronic structure and spectra of the MoN molecule

The electronic absorption spectra of the MoN molecule have been measured by intracavity laser spectroscopy, and its electronic structure has been calculated by the X..cap alpha..-SW method. The absorption spectra showed 69 quanta (of which 55 were obtained for the first time), which were assigned on the basis of their isotope shifts to the MoN molecule. The energies and oscillator strengths of the one-electron transitions to states lying below approx. 4 eV from the ground and the lowest excited electronic states were calculated in the transition-state approximation. The results are presented.