Concentration-modulated Absorption Spectroscopy Research Articles

Absorption spectrum of neutral krypton in the near infrared region

High-resolution absorption spectra of atomic krypton in the range of 11870-12700 cm $^{-1}$ were recorded by employing concentration modulation absorption spectroscopy technique with a tunable single-mode cw Ti:Sapphire laser. The krypton atoms were excited to the absorbing energy states by discharge-burning in a mixture of helium and krypton. A total of 120 lines of neutral krypton were observed, among them 33 lines had already been classified in previous studies, 45 lines were newly classified with the known energy levels, and 42 lines cannot be classified. These unclassified lines indicate that up to now unknown energy levels of Kr must exist. Further, an analysis of the unclassified lines to get possible new energy levels with a classification program is reported.

Hyperfine structure of atomic fluorine (F I)

A high resolution absorption spectrum of neutral fluorine(F I) was observed around 800 nm using concentration modulation absorption spectroscopy with a tunable Ti : Sapphire laser. The fluorine atoms were produced by discharging the mixed gases of helium and sulfur hexafluoride (SF6) in a glass tube. Thirty four hyperfine structure (hfs) resolved transitions were analyzed to obtain 23 magnetic dipole hfs constants A for 2p4(3P)3s, 2p4(3P)3p and 2p4(3P)3d configurations. The hfs constants in 2p4(3P)3s and 2p4(3P)3p configurations were compared with those obtained from experiments and calculations. Fifteen constants in 2p4(3P)3d configuration were reported - to our knowledge - for the first time.

Hyperfine structure constants of atomic bromine (Br I)

The absorption spectrum of the neutral bromine (Br I), lying in the region from 11,300cm−1 to 12,600cm−1 has been investigated using concentration modulation absorption spectroscopy with a tunable Ti:Sapphire laser. The bromine atoms were excited by discharging the mixture of Helium and bromine vapour in a hollow discharge glass tube. The hyperfine structure spectra of 45 lines were analyzed. Hyperfine structure constants A and B were derived for 20 even and 27 odd levels amongst which constants for 12 even and 21 odd levels were newly reported.

Hyperfine structure near infrared spectrum of atomic iodine

The hyperfine structure of spectral lines of neutral iodine (II) in the wavenumber region from 11300cm−1 to 13000cm−1 was studied using optical heterodyne concentration modulation absorption spectroscopy with a tunable single-mode cw Ti:Sapphire laser. The iodine atoms were produced and excited by a discharge operated in a mixture of helium and iodine vapor. A total of 89 lines were observed and the hyperfine structure of 45 of these lines was studied. For 40 lines, the hyperfine structure was not previously known. The magnetic dipole and the electric quadrupole hyperfine structure constants of 18 even and 28 odd levels were extracted. The constants of one even level and of 16 odd levels are presented for the first time.

Absorption Spectroscopy of (8,1) Band in d3Δ-a3Π System of CS Radical

The CS radical was generated by discharging the mixture gas of CS2 and Helium. The Doppler limited spectra of CS were recorded in the region of 12350–12950 cm−1 using optical heterodyne concentration modulation absorption spectroscopy. Three hundred and twenty-six lines were recorded and assigned to the d3Δ-a3Π (8,1) band, in which eighty-three transitions were first observed. A set of improved molecular constants for the d3Δ(v=8) and a3Π(v=1) levels were determined by a non-linear least-squares fitting of all the lines to the effective Hamiltonian.

Absorption spectrum of the (2, 1) band in the A2П i– X2Σ + system of CS + cation

The spectrum of CS + cation was recorded in the 12,235–12,600 cm −1 region by employing optical heterodyne velocity modulation absorption spectroscopy. In combination with simultaneous measurement of optical heterodyne concentration modulation absorption spectroscopy, the spectral lines of CS + blended with the neutral CS lines were identified and extracted from the observed spectra. One hundred and eighty-two spectral lines of CS + were assigned to the (2, 1) band in the A 2П i – X 2Σ + system and the improved molecular constants of the upper level ( υ = 2) in the A 2П i state were derived by nonlinear least squares fitting.

Concentration-modulated absorption spectroscopy and the triplet state: Photoinduced absorption/bleaching in erythrosin B, rose bengal and eosin y

The pump and probe method concentration-modulated absorption spectroscopy has been applied for studying the triplet characteristics of dyes erythrosin B, rose bengal and eosin y in ethanol solutions. Using the 514.5 and 488 nm lines of an argon ion laser for pump and probe beams, respectively, photoinduced bleaching for erythrosin B and eosin y and photoinduced absorption for rose bengal have been observed. The theory developed for describing the interactions of dye molecules of strong triplet forming ability with continuous wave pump and probe beams of Gaussian beam profiles has been tested by analysing the probe power gain or loss at different positions along the optical path of the focused collinear beams. From the gain–modulation frequency curves triplet absorption cross sections and the lifetimes of the triplet states have been determined.

Production, thermalization, and transport of sputtered lithium atoms across a glow discharge plasma, studied by concentration modulated absorption spectroscopy

Sputtered Li atom density profiles have been measured, absolutely, using concentration modulated absorption spectroscopy (COMAS), across a planar Ar glow discharge. They are found to fit very well to a simple analytical solution to the sputtering model, previously proposed, of atom recoil from the surface and thermalization as the kinetically energetic atoms are stopped by collision and subsequent diffusive transport. “Stopping” or thermalization cross sections have thus been experimentally measured and are found to be a factor of between four and eight times less than the hard sphere collision cross section, depending on the recoil energy. The actual particle flux bombarding the cathode and causing sputtering is shown to be greater than the ion flux, depending on conditions.

Temporal and spatial distribution profiles of lithium atoms in low-pressure discharges by concentration-modulated absorption spectroscopy

The application of the new pump–probe spectroscopic technique, COMAS (concentration-modulated absorption spectroscopy), to the study of sputtered lithium atoms in two glow-discharge devices is discussed. Information is presented on the absolute concentration of lithium atoms in a hollow cathode lamp (1.62 × 1011 cm–3 in the centre at 14 mA) and a plane-parallel dc discharge (1.24 × 109 cm–3 close to the cathode at 1.05 mA). In addition an experimentally derived absorption cross-section for the (2s)2, S →(2p)2, P1/2, 3/2 resonance transition in lithium (σ≈ 2.95 × 10–12 cm2) is presented for the first time. In producing concentration profiles for both a commercial hollow cathode lamp and a glow-discharge system built in-house, use is made of the high spatial resolution attainable with COMAS. Results obtained in the dc plane-parallel system are compared to a theoretical model describing the transport of sputtered atoms through a low-pressure glow discharge. By varying the delay between the pump and probe laser pulses, the lifetime of the (2p)2, P1/2, 3/2 state has been measured as 25 ± 1 ns showing good agreement with values obtained by other methods found in the literature (ca. 24–27 ns).

Concentration-modulated absorption spectroscopy. Part 5.—Influence of molecular diffusion

The absorption of laser radiation by a chromophore creates a non-equilibrium distribution of excited-state species whose temporal decay back to the equilibrium distribution can be monitored by the familiar techniques of pump–probe spectroscopy. When this anisotropic distribution is created in a highly localised region at the focus of a laser beam and when the excited-state lifetime is suitably long, molecular diffusion provides an additional decay route for the recovery of the ground-state population in that region of space interrogated by the probe beam. The influence of this spatial decay in space and time on the population difference between the two electronic states coupled by the pump and probe laser beams is investigated theoretically and experimentally using the technique of concentration-modulated absorption spectroscopy. A modified form of the ‘gain’ expression of our earlier studies is proposed which is shown to account satisfactorily for the form of the ‘gain’vs. the logarithm of the modulation frequency curves observed experimentally. Although thermal lensing plays a part in influencing the transmission characteristics of the probe laser it is apparent that molecular diffusion is the dominant process in the experimental arrangement employed. The potential value of this technique for the study of molecular diffusion in highly localised regions of space is emphasised.

Concentration-modulated absorption spectroscopy: profiles of the Na D lines in a hydrocarbon flame

The potential value of pump-probe spectroscopy for the study of the spectral profiles of narrow gas-phase absorption lines has been investigated using the technique of concentration-modulated absorption spectroscopy (COMAS). The relationship of the probe laser 'gain' to the absorption line profile is established and the parameters of the Voigt line profile for the 3p2P12 /-3s2S12 / transition of atomic sodium in a hydrocarbon flame have been determined. The effects of a significant reduction in the Doppler contribution to the linewidth of this transition in the flame are clearly identified.

Concentration-modulated absorption spectroscopy. Lifetime measurements of the 2E state of Cr3+ in a ruby crystal

The quantitative pump–probe technique of concentration-modulated absorption spectroscopy is employed to measure the concentration of the Cr3+ ion in a ruby crystal. A previously developed theory, confirmed experimentally in this study, shows how measurements of the fractional power change of a probe laser at various frequencies of modulation of pump laser can provide information on state lifetimes of absorbing chromophores. The measured lifetime of the 2E state of Cr3+ in the crystal (2.9 ms) is used with absorbance data to provide a direct measure of the Cr3+ concentration in the crystal.

Concentration-modulated absorption spectroscopy. Part 3.—The effect of finite spectral linewidths

Finite spectral linewidths of radiation sources in absorption spectroscopy lead to marked deviations from the Beer–Lambert relationship when quantitative studies are carried out on spectrally narrow transitions. Such deviations are even more marked in concentration-modulated absorption spectroscopy, a newly developed quantitative technique which takes advantage of the high power of laser sources to perturb the concentrations of absorbing species in localized regions of space. However, when advantage is taken of the formal relationship which exists between probe laser gain in concentration-modulated absorption spectroscopy and sample absorbance in order to determine species concentration, the results appear to be free of the effects of source linewidth. The reasons for this surprising observation are accounted for quantitatively in this study and it is shown that even for a laser linewidth an order of magnitude broader than the absorption line, little error is introduced in the measured sample concentrations. Since the analysis also demonstrates a method for correcting experimental absorbance data for the influence of finite resolution effects, it is possible to use the measured concentrations with the corrected absorbance data to determine accurate values for absorption cross-sections.

Concentration-modulated absorption spectroscopy. Part 4.—The use of continuous-wave lasers

The concentration-modulation approach to the study of absorption spectra of atoms and molecules developed with mode-locked dye lasers is extended in this paper to continuous-wave lasers. A ‘gain’ theory is developed for use with c.w. lasers and a comparison is made between the ‘gain’ measurements of our c.w. experiments and those of earlier studies using pulsed lasers. The experimental agreement between the measured c.w. ‘gain’ and the pulsed ‘gain’, after correcting for differences between the experimental arrangements employed, provides strong support for the ‘gain’ theory developed here. It is clearly seen that by means of this theory and the derived equations it will be possible to evaluate species concentrations where the state lifetimes are known, or the lifetimes where the concentrations are known. These studies provide a firm foundation for expansion of the applicability of this highly sensitive absorption technique for kinetic and spectroscopic investigation of chemical systems.

Concentration-modulated absorption spectroscopy. II. temporal variation of gain

Abstract The application of concentration-modulated absorption spectroscopy, COMAS, to the determination of lifetimes of the electronic states of absorbing species is discussed. The theory behind the lifetime calculations is described and experimental results for lithium in a hydrocarbon flame are reported. The lifetime of the 2p 2 P 3 2 , 1 2 and 3d 2 D 5 2 , 3 2 states are respectively measured as 528±20 and 216±20 ps for lithium atoms in an ethyne—air flame. The power of the technique in measuring accurately the lifetimes of high-lying thermally excited state species is discussed and areas for further work are suggested.

Concentration-modulated absorptiion spectroscopy.I

Abstract The absorption of photons from a high-intensity beam of laser radiation perturbs the concentration difference between the two states coupled by the incident radiation. This perturbation is investigated by a second beam using well-known pump and probe methods. By using picosecond mode-locked lasers and high-frequency modulation methods the sensitivity limit of the technique developed, termed concentration-modulated absorption spectroscopy, COMAS, is extended dramatically and compares most favourably with fluorescence excitation spectroscopy. The quantitative treatment of the theory shows that the gain created on the probe laser can be related formally to the transmittance of the sample, the relationship providing a direct measure of the concentration of the absorber. This COMAS approach to concentration measurement dispenses with the need for calibration using a known concentration of the chromophore or an accurate knowledge of the appropriate absorption coefficient. Accordingly, it is most suitable for investigating species concentrations in mixtures in the gaseous and in the liquid states. Experimental results using lithium in a hydrocarbon flame and thionin (Lauth's violet) in aqueous solution are reported. Some of the wide ranging applications of this technique are discussed.