High resolution pulsed infrared cavity ringdown spectroscopy: Application to laser ablated carbon clusters

A BaGa4Se7 crystal based pulsed mid-infrared light source with a narrow linewidth in 4-12 µm

We present a BaGa4Se7 (BGSe) crystal based coherent pulsed light source for high resolution mid-infrared (MIR) spectroscopy in the 4-12 µm region. The all-solid-state system consists of an injection seeded optical parametric generator (OPG) and an optical parametric amplifier (OPA) using two KTiOPO4 crystals. The idler output of OPG-OPA and the fundamental output (1064 nm) of a wavelength stabilized Nd:YAG laser are employed for difference frequency generation of MIR pulses in the BGSe crystal. Pulsed MIR radiation in the 4-12 µm range is obtained with typical pulse energies higher than 100 µJ and pulse durations of ∼5 ns. By measuring H2O absorption lines in the 8 µm region with this MIR light source and a cavity ring-down spectrometer, the linewidth of the MIR source is inferred as 120 ± 10 MHz, which is very close to the Fourier-transform limited linewidth of 5 ns laser pulses.

Influence of Spatial Inhomogeneity of Detector Temporal Responses on the Spectral Fidelity in Continuous Wave Cavity Ringdown Spectroscopy

Due to their advantages of having a wide bandwidth, low cost, and being easy to obtain, traditional photodetectors (PDs) are being widely applied in measurements of transient signals. The spatial inhomogeneity of such PD temporal responses was measured directly to account for the PD spatial effect of decay rate due to poor alignment in continuous wave cavity ringdown spectroscopy (CW-CRDS) experiments. Based on the measurements of three PDs (i.e., model 1611 (Newport), model 1811 (Newport), and model PDA10CF-EC (Thorlabs)), all the temporal responses followed a tendency of declining first and then rising, and steady platforms existed for the last two PDs. Moreover, as we expected, the closer the PD center was, the faster the response. On the other hand, the initial shut-off amplitude generally reached a larger value for a faster temporal response. As a result, the spatial effect can strongly influence the spectral line shape and value, which will introduce more errors into the precise measurements of spectral parameters using the CRDS technique if this effect is not considered. The defined effective detection area (EDA) of the PDs, which was close to the active area given by manufacturers, was the key parameter that should be paid more attention by researchers. Therefore, the PD should be aligned perfectly to make sure that the EDA covers the laser spot completely.

Isotopic study of new fundamentals and combination bands of linear C5, C7, and C9 in solid ar

A high yield of carbon chains has been produced by the laser ablation of carbon rods having (13)C enrichment. FTIR spectroscopy of these molecules trapped in solid Ar has resulted in the identification of two new combination bands for linear C(5) and C(9). The (ν(1) + ν(4)) combination band of linear C(5) has been observed at 3388.8 cm(-1), and comparison of (13)C isotopic shift measurements with the predictions of density functional theory calculations (DFT) at the B3LYP/cc-pVDZ level makes possible the assignment of the ν(1)(σ(g)(+)) stretching fundamental at 1946 cm(-1). Similarly, the observation of the (ν(2) + ν(7)) combination band of linear C(9) at 3471.8 cm(-1) enables the assignment of the ν(2)(σ(g)(+)) stretching fundamental at 1871 cm(-1). The third and weakest of the infrared stretching fundamentals of linear C(7), the ν(6)(σ(u)(+)) fundamental at 1100.1 cm(-1), has also been assigned.

Cavity ringdown spectroscopy in a hollow Bragg waveguide: electromagnetic theory and modeling

Cavity ringdown spectroscopy (CRDS) is a gas sensing technique in which an optical cavity is formed by two or more highly reflective mirrors. Herein we present an overview and historical perspective of CRDS implementations that seek to reduce or eliminate some of the disadvantages of conventional CRDS by modifications to the ringdown cavity (RDC). The hollow waveguide (HWG) CRDS concept that we introduce in this paper reduces some of the disadvantages of conventional CRDS by utilizing a hollow waveguide as the RDC. We develop the basic mathematical theory and model for the HWG-CRDS concept and provide an initial in-depth study of the Bragg waveguide for CRDS applications. We also discuss various aspects of design and performance characteristics of HWG-CRDS, including waveguide attenuation losses with and without gases in the waveguide core, transverse and longitudinal mode propagation behavior, and methods and analysis for the HWG-CRDS excitation.

Methyl iodide clusters observed in gas phase by infrared cavity ring-down spectroscopy: The CH3 bending mode at 8μm

Infrared spectra of methyl iodide clusters produced in a supersonic jet have been observed in the CH3 bending region at 8 mum by cavity ring-down spectroscopy. The dependence of the spectral features on the mixing ratio of CH3I to He and on the stagnation pressure has allowed us to assign the absorption peaks, with the help of the previous results obtained by matrix-isolation technique [Ito et al., Chem. Phys. Lett. 343, 185 (2001)] and infrared cavity ring-down spectroscopy in the C-H stretching region [Ito et al., Chem. Phys. 286, 337 (2003)]. Ab initio calculations at the MP2 level have been carried out up to tetramer to confirm the assignments. It has been found that the frequency shifts upon clustering (relative to monomer) observed in the bending region are not monotonic, in contrast to those in the C-H stretching region. The observed frequency shifts are discussed in terms of dispersion interaction and its variation upon vibrational excitation.

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.

Generation of infrared radiation by stimulated Raman scattering in para-hydrogen crystal at 5 K

We report the preliminary results of our experiments with stimulated Raman scattering in para-hydrogen crystal aimed at developing a continuously tunable laboratory laser source of mid-infrared radiation. With laser pulses at 532 nm, a conversion efficiency for the first Stokes beam of as much as 20% in the forward direction was observed through a single-pass, 5-cm-long crystal. Generation of mid-infrared pulses at 4.5 microm was achieved by use of the output of a near-infrared pulsed laser (1.6 microm, 3 ns), and an absorption spectrum of gaseous CD4 molecules was successfully recorded. These results suggest use of the solid para-hydrogen Raman shifter as a promising light source for mid-infrared spectroscopy.