Infrared Diode Laser Spectroscopy Research Articles

CO internal excitation in the photodissociation of Cr(CO)6 at 278 nm

Time-resolved infrared diode laser spectroscopy has been used to probe CO internal excitation in the photodissociation of Cr(CO)6 at 278 nm. At the high fluence level of these experiments, CO state distributions are not consistent with a statistical model of single-photon absorption. A simple Franck—Condon treatment, assuming multiphoton absorption into a dissociative continuum, qualitatively describes the observed CO vibrational distribution.

Kinetics of the addition reaction of methyl radicals with nitric oxide studied by pulse radiolysis combined with infrared diode laser spectroscopy

The reaction CH 3+NO (+M) → CH 3NO (+M) was initiated by pulse radiolysis of acetone/nitric oxide mixtures and the kinetics of methyl radicals was studied by time-resolved infrared absorption spectroscopy. The rate constant was found to be strongly pressure dependent in the range of p(M) = 6.5−150 mbar at 298 K with M = acetone as the third body. The experimental results are represented in terms of a fall-off curve centered at 37 mbar with limiting high- and low-pressure rate constants of k rec,∞=(6.6 ± 0.9) × 10 9 × ( T/300) 0.6 M −1 s −1 and k rec,0/[M] = (4.4 ± 0.4) × 10 12 × ( T/300) −3.5 M −2s −1, respectively.

Kinetics of the reactions H + C 2H 4 → C 2H 5, H + C 2H 5 → 2CH 3 and CH 3 + C 2H 5 → products studied by pulse radiolysis combined with infrared diode laser spectroscopy

Formation of methyl radicals via the consecutive reactions H + C 2H 4 + M → C 2H 5 + M (1) and H + C 2H 5 → CH 3 + CH 3 (2a) was initiated by pulse radiolysis of 10–100 mbar H 2 in the presence of ethylene. The kinetics of CH 3 were studied by monitoring the transient infrared absorption at the Q(3, 3) line of the ν 2 = 0 → 1 vibrational transition at 606.12 cm −1. Reaction (2a) proceeds in competition with the reaction H + C 2H 5 + M → C 2H 6 + M (2b). The methyl radicals are consumed by CH 3 + CH 3 + M → C 2H 6 + M (3), CH 3 + C 2H 5 → products (4), and H + CH 3 + M → CH 4 + M (5). By computer simulations based on determined absolute radical yields, the observed formation and decay kinetics could be reproduced using the rate constants k 2a = (7.5 ± 0.7) X 10 10 M −1 s −1, k 2b = (1.0 ± 0.1) X 10 11 M −1 s −1, and k 4 = (4.0 ± 0.2) X 10 10 M −1 s −1 at 298 K and a total pressure of p(H 2) = 100 mbar.

High-resolution diode laser spectroscopy of the ν16 band of N-methylmethylenimine

High-resolution infrared diode laser spectroscopy of molecular discharges through amines and diamines has been carried out. The IR spectrum of the ν 16 band of the short-lived species CH 3N:CH 2 ( N-methylmethylenimine) has been recorded at the 10-μm region with a resolution of 0.002 cm −1. Over 100 vibration-rotational transitions belonging to the ν 16 (CH 2 rocking) fundamental band of CH 3N:CH 2 have been assigned near the band center at 1026 cm −1. The rotational constants of the excited state of this C-type band have been determined by using a least-squares fitting programme with a standard deviation of less than 0.001 cm −1.

High-resolution infrared diode laser spectroscopy of SO( 3Σ −) in a secondary-slit supersonic expansion

A two-stage pulsed expansion method is reported in which free radicals are generated by laser photolysis just prior to expansion. The technique is applied to the 2←0 vibrational overtone band of SO(X 3Σ −). Essentially complete SO 2 photolysis is achieved. Following expansion, SO rotational temperatures are between 16 and 50 K depending on the SO 2 concentration.

Direct kinetic studies of SiH3+SiH3, H, CCl4, SiD4, Si2H6, and C3H6 by tunable infrared diode laser spectroscopy

Gas phase reactions of silyl radical, SiH3, are investigated at room temperature using tunable diode laser flash kinetic spectroscopy. Photolytic generation of silyl at 193 and 248 nm is demonstrated using several different precursor systems. The silyl recombination reaction, SiH3+SiH3→Si2H6, is studied by quantitative measurement of SiH3 and attendant product densities. Analysis yields a refinement of the rate constant, krc=(7.9±2.9)×10−11 cm3 molecule−1 s−1. By modeling silyl densities following photolysis of HCl in SiH4, bimolecular rate constants for H+SiH3 and H+SiH4 are determined to be (2±1)×10−11 and (2.5±0.5)×10−13 cm3 molecule−1 s−1, respectively. Reactions of SiH3 with SiD4, Si2H6, CCl4, and C3H6 (propylene) are studied under pseudo-first-order conditions. Derived upper limits to the rate constants show these reactions to be slow at room temperature. The data demonstrate the reactivity of silyl with open-shell (radical) species and the general inertness of silyl toward closed shell molecules. Under typical chemical vapor deposition conditions, SiH3 is, therefore, a kinetically long-lived species in the gas phase and consequently a potentially important film forming species under plasma and photochemical deposition conditions.

Infrared diode laser spectroscopy of the electronic ground state of SnD

We report the first direct observation of the vibration-rotation spectrum of the tin deuteride radical in its electronic ground state 2Π by infrared diode laser spectroscopy. The SnD radicals were produced using tetramethylstannane (Sn(CH3)4) and deuterium in a DC helium glow discharge. The isotopic structure, the lambda-type doubling and the hyperfine splitting due to the tin nucleus could be resolved. A set of molecular parameters for the ground state is given.

Infrared diode laser spectroscopy of the GeH radical in the 2Π1/2 state

The vibration–rotation spectrum of the GeH radical for the fundamental band in the 2Π1/2 spin state was observed using an infrared diode laser spectrometer. The GeH radical was produced by ac glow discharge in the mixture of GeH4, He, and H2. The signals were detected by the discharge modulation method. In total 94 GeH lines were identified for five isotopic species, and were simultaneously analyzed to determine effective molecular constants in the 2Π1/2 spin state.

Infrared diode laser spectroscopy of the ground state of GeH (X 2Π1/2)

Transitions in the fundamental band of the ground state X 2Π1/2 of the GeH radical are detected by tunable diode laser spectroscopy. Signals are observed for all five naturally occurring isotopes of germanium. A preliminary fit of the vibrational-rotational constants for the ground state is given, in which the vibrational corrections α A , αγ and α p+2q to the fine structure constant A, the spin rotation constant γ and the lambda doubling constant p + 2q are determined from our data.

Tunable diode laser spectroscopy for isotope analysis-detection of isotopic carbon monoxide in exhaled breath

A high resolution tunable infrared diode laser spectroscopy system was developed for isotope analysis with sensitivity at ppb levels. Such a system is ideally suited for detection and measurement of minute amounts of infrared active compounds present in a huge noninfrared active background such as air. The operation and capabilities of the system were demonstrated by measuring physiological levels of isotopic carbon monoxide, 12C16O and 13C16O, naturally present in exhaled human breath with essentially no sample preparation. The simplicity in obtaining such data suggests that fundamental physiological information may be derived from noninvasive measurements. This makes the system potentially useful for many biomedical applications.

The C9 cluster: structure and infrared frequencies.

The high resolution infrared spectrum of the C9 cluster has been measured in direct absorption by infrared diode laser spectroscopy of a pulsed supersonic carbon cluster jet. Fifty-one rovibrational transitions have been assigned to the ν6 (σu ) antisymmetric stretch fundamental of the 1Σ+9 linear ground state of C9. The measured rotational constant [429.30(50) MHz] is in good agreement with ab initio calculations and indicates an effective bond length of 1.278 68(75) Å, consistent with cumulenic bonding in this cluster. Several perturbations are observed in the upper state, and the upper- and lower-state centrifugal distortion constants are observed to be anomolously large, evidencing a high degree of Coriolis mixing of the normal modes.

Infrared diode laser diagnostics of methane plasmas produced in a deposition reactor

Tunable infrared diode laser spectroscopy has been applied to the study of CH4 plasmas generated in a deposition reactor run at 20 kHz. Absolute number densities of CH4, C2H4, and CH3 have been measured as functions of pressure and current. A qualitative explanation of the results is presented.

CO on Pt(335): Vibrational Stark effect, mode coupling, and local field effects on a stepped surface

We have studied CO on Pt(335) using infrared diode laser spectroscopy and electric field modulation. The spectra cover the 2000–2160 cm −1 range. The Pt(335) surface is stepped, with (111) terraces four atoms wide. At low coverage only one species is observed: CO adsorbed on step edges. In terms of the applied field, the Stark tuning rate of a low coverage of this species is (6.0 ± 0.8) × 10 −7 cm −1/( V/ cm). At the same coverage, the screened dynamic dipole moment ~ e = (0.94 ± 0.11) e. The rate calculated from ~ e is (7.0 ± 1.1) × 10 −7 cm −1/( V/ cm), agreement with the measured value. At higher coverages te also observed. Terrace CO is similar to CO on Pt(111) in vibrational frequency, dynamic dipole moment and desorption temperature. However, the Stark tuning rate of terrace CO is at least an order of magnitude smaller than expected. We discuss why the tuning rate might be so small. When both species are present, the vibrational spectra are strongly affected by dipole-dipole coupling. As a result, the electroreflectance spectrum is not the derivative of the polarization modulation spectrum. The spectra are analyzed using a phenomenological coupled-oscillator model. Fitting the spectra to the model, we find that for CO adsorbed on step edges, the Stark tuning rate in terms of local field is(7.5 ± 2.0) × 10 −7 cm −1/( V/ cm), independent of coverage. The saturation coverage clean surface at 300 K is (8.3 ± 0.6) × 10 14 cm −2. With saturation coverage of CO the linewidth is very small, only 4.5–5.0 cm −1 FWHM, even though the sample is at 300 K. Also, while cleaning the surface, we observed a correlation between Sn coverage and saturation coverage of CO. One possible explanation for the correlation, although not proven, is that Sn blocks CO adsorption sites. If so, the data suggests that each Sn atom blocks 11 ± 6 CO adsorption sites, which may explain how Sn helps prevent CO poisoning of Pt electrodes in fuel cells.

Infrared diode laser spectroscopy of SCl generated by the photolysis of S 2Cl 2 and SCl 2

The photolysis of S 2Cl 2 and SCl 2 with excimer laser radiation at 248 nm yields the SCl radical in its electronic ground state X 2 Π 3 2 . By infrared diode laser kinetic spectroscopy the vibrational bands v = 1-0 and 2-1 have been observed to high rotational levels ( J ≤ 59.5) and the spectra of the isotopic species 32S 35Cl and 32S 37Cl have been assigned. The spectrum is fitted to a few molecular parameters using isotope relations. It cannot be ruled out that the hot band (2-1) should be assigned to the missing X 2 Π 1 2 state.

Rate constant measurements for reactions of SiH3 with O2, NO and NO2 using time-resolved infrared diode laser spectroscopy

The rate constants for the reactions SiH3+O2, SiH3+NO2 and SiH3+NO+N2 have been measured over the pressure range 1–10 Torr at 300 K, by monitoring a SiH3 absorption line at 719.931 cm−1 using time-resolved diode laser spectroscopy. SiH3 radicals were prepared by pulsed CO2 laser photolysis of CCl3F in SiH4: CCl3F + nhv → CCl2F + Cl, Cl + SiH4 → HCl + SiH3. Analysis of the pseudo-first-order decay of SiH3 provided rate constants of (1.3 ± 0.3) × 10−11 and (5.1 ± 0.9) × 10−11 cm3 molecule−1 s−1 (±2σ) for the reactions with O2 and NO2 respectively and 8.2 ± 0.9 × 10−30 cm6 molecule−2 s−1 (±2σ) for the reaction SiH3 + NO + N2.

Transient molecules, free radicals and molecular ions, investigated by high-resolution spectroscopy

Abstract High-resolution spectroscopic techniques, in particular microwave and infrared diode laser spectroscopy, have been applied to detect, identify and characterize transient molecules such as free radicals and molecular ions in the gas phase through the observation of their rotational, rovibrational and/or rovibronic spectra. Some recent results are discussed in detail. Kinetic spectroscopy has been developed in the infrared and microwave regions to clarify chemical and physical processes involving transient molecules; photochemical and photo-induced reactions are investigated by recording the spectra of intermediate species with this technique in real time. Two applications of spectroscopy of transient molecules are mentioned; one is detection and identification of interstellar molecules and the other is development of a means for diagnosing discharge plasmas such as those employed for making thin films of amorphous silicon.

Quantum yield studies of disilane photodissociation at 193 nm by infrared diode laser spectroscopy

Quantum yields for the loss of disilane and the formation of silane in the 193 nm photodissociation of disilane have been measured using time-resolved infrared diode laser absorption spectroscopy under single excimer laser pulse conditions at total pressures of 5 to 10 Torr in helium buffer gas. The total quantum yield for loss of disilane is 0.7±0.1 while the quantum yield for formation of silane is only 0.10±0.03. The results suggest that numerous photodissociation pathways as well as non-photodissociative relaxation pathways exist for disilane excited near its electronic absorption threshold.

Infrared diode laser spectroscopy of triacetylene by the source and the Stark modulation techniques

Abstract The infrared diode laser spectrum of the ν11 (πu: antisymmetric CCH bending) band of triacetylene (HCCCCCCH) was observed by the source and the Stark modulation techniques. Precise rotational constants were derived for the ground and ν11 vibrational states. The ν11 band origin was determined to be 621.33998(23) cm−1 with the standard deviation given in parentheses. The Stark modulation spectrum showed a strong feature around the Q-branch head, its lineshape indicating that the ν8 (πg: symmetric CCH bending) state lies above the ν11 state. Analysis of the Stark modulation spectrum with the line profile simulation allowed us to deduce the origin of the infrared inactive ν8 band, 622.38 ± 0.40 cm−1.

Infrared diode laser spectroscopy of BrF and BrCl

Diode laser spectra of BrF and BrCl below 700 cm −1 were observed and analyzed. Transitions in the 1-0 and 2-1 bands of 79 BrF and 81 BrF and those in the 1-0, 2-1, and 3-2 bands of 79 Br 35 Cl, 81 Br 35 Cl, 79 Br 37 Cl, and 81 Br 37 Cl were fitted to seven and eight Dunham coefficients, respectively. Many BrCl lines, which can be used for wavenumber calibration of diode laser spectra, were obtained between 400 and 450 cm −1 with an accuracy of about 0.001 cm −1 .

Infrared diode laser spectroscopy of the ν2 + ν3 band of CCH

A spectrum of a 2Π- 2Σ transition has been observed in the wavenumber region 2088–2130 cm −1 by a discharge in an acetylene and hydrogen mixture and is assigned to the ν 2 + ν 3 combination band of CCH. The vibrational frequency of this band (2091 cm −1) is much lower than the sum 2212 cm −1 of the ν 2 (370 cm −1) and ν 3 (1841 cm −1) fundamental frequencies. This low frequency and a large change in the effective spin-orbit interaction constant from the v 2 = 1 state are explained by a vibronic interaction between the A ̃ 2 Π and X ̃ 2 Σ states.