Pulsed Slit Jet Expansion Research Articles

High-Resolution Continuous-Wave-Diode Laser Cavity Ring-Down Spectroscopy of the Hydrogen Fluoride Dimer in a Pulsed Slit Jet Expansion: Two Components of the N = 2 Triad near 1.3 μm

The near-infrared overtone spectra of the prototypical hydrogen-bond hydrogen fluoride dimer (HF)2 contain rich information on hydrogen bond dynamics. We report a study of the N=2 triad involving excitations with two quanta of HF stretching in (HF)2 around 1.3 microm (7500-7800 cm(-1)) by means of continuous-wave-diode laser cavity ring-down spectroscopy in a pulsed supersonic slit jet expansion. The analysis of the rotationally resolved overtone spectra allows the study of vibrational mode-selective kinetics, such as hydrogen bond predissociation with lifetimes tauPD and tunneling rearrangement (switching) processes with periods tausw obtained from the tunneling splitting DeltaT in highly excited vibrational states. The Ka=1<--0 transition of the Nj=22 band of (HF)2 has been reinvestigated by us in a supersonic jet expansion; the much improved data obtained here are in excellent agreement with several previous experimental results. Our analysis provides subband-level positions and properties 0(Ka=1(A+))=7711.37956(66) cm(-1), DeltaT=0.0936(10) cm(-1), and tauPD=1.3-1.9x10(-9) s, depending on the level symmetry A+ and B+. We have also analyzed spectra of the Nj=21 band, which we have observed for the first time in a supersonic jet with rotational resolution. For the Ka=0<--0 transition of this band, we find the band center at 0(A+)=7550.3555(26) cm(-1) and a tunneling splitting of DeltaT=0.0150(37) cm(-1). This level involves mostly excitation of the H-bonded HF stretching with two quanta. The mode-selective tunneling switching is in agreement with a simple picture of inhibited tunneling. These experimental values are close to those calculated on the "SO(-3)" potential energy hypersurface of Klopper, Quack, and Suhm. The N=2 triad also exhibits a strongly mode-selective predissociation dynamics, with a predissociation lifetime tauPD=4.99(84)x10(-11) s in the Nj=21 level, which is more than 20 times shorter than that for the Nj=22 level.

Rovibrational analysis of the ν1fundamental of CF3Cl from diode laser spectra in a supersonic slit-jet expansion

Spectra of the ν1 fundamental band of CF3Cl (CFC-13), with natural isotopic abundance, were recorded with a tunable diode laser spectrometer in the range 1100–1116 cm−1 using a supersonic pulsed slit-jet expansion. A rotational temperature of about 50 K was reached and a significant simplification of the spectra was achieved. The reduced Doppler broadening obtained using the slit source led to a FWHM up to 0.0007 cm−1 and so it was possible to resolve almost completely the K-structure in the P(J) and R(J) clusters for the first time. From the final fit of the assigned transitions a reliable set of molecular constants including the quartic centrifugal distortion coefficients for the v1 = 1 state of CF335Cl and CF337Cl has been obtained.

High-resolution infrared spectra of the C-H asymmetric stretch vibration of jet-cooled methoxy radical (CH3O)

The high-resolution infrared spectrum of jet-cooled methoxy radical (CH3O) in the region 2850–2940 cm−1 has been recorded at about 26 K in a pulsed slit-jet expansion producing the radical by excimer laser flash photolysis. A perpendicular band centered near 2885 cm−1 was assigned to a portion of the C-H asymmetric stretch vibration (ν4) of CH3O by means of ground-state combination differences. Five ΔP=+1 subbands with P″=−1.5, −0.5, 0.5, 1.5, 2.5, and J″ up to 9.5 have been assigned. Because of the low jet temperature, the assigned transitions all arise from the ground E3/2 fine-structure component of the lowest vibronic state. Preliminary analysis yields values of A, B, and D of 5.232(12), 0.913(8), and 2.5(1.5)×10−5 cm−1, respectively. A small hyperfine splitting was observed in the P″=0.5 series transitions.

High-resolution spectroscopy of the ν3 band of WF6 and ReF6 in a supersonic jet

We have recorded the Fourier-transform infrared (FTIR) spectrum of the ν3 fundamental band of WF6 in a continuous supersonic jet expansion with an instrumental bandwidth of 0.0024 cm−1 (FWHM, full width at half maximum, unapodized), using a Bomem DA.002 spectrometer. Some parts of this band have also been recorded with 0.0007 cm−1 bandwidth using a diode laser spectrometer combined with a pulsed slit jet expansion. A multiple-pass arrangement has been used for the slit jet to observe low-intensity lines. In each case, we have used a WF6:He mixture with a seeding ratio 1:3 leading to a rotational temperature of ca. 50 K. This work extends the previous investigation of Takami and Kuze [J. Chem. Phys. 80, 5994 (1984)] to much higher J transitions. In both P and R branches, rotational lines have been recorded for J up to 46–48. We have used a tensorial Hamiltonian adapted to the group chain O(3)⊃Oh and developed to the third order for the analysis of the spectra. A least-squares fit for each of the four main isotopic species: 182WF6, 183WF6, 184WF6, and 186WF6 results in band centers (in this order) 714.538 19, 714.214 06, 713.895 44, and 713.266 21 cm−1. We report furthermore first results on the high-resolution spectra of ν3 of ReF6, which exhibits a fourfold degenerate electronic ground state of Gg′ species in the OhS group. Supersonic jet-FTIR spectra show a moderately structured relatively broad band, whereas the diode laser spectroscopy of the seeded jet in the range 708–733 cm−1 results in line resolved spectra of high complexity. A preliminary analysis is discussed, while a complete analysis still represents an appreciable challenge.

Construction and characterization of a short pulse slit-jet source based upon the current loop-actuated design

A pulsed slit-jet expansion source capable of generating moderate velocity-collimated pathlengths (∼1 cm) of rovibronically cooled target molecules for spectroscopic interrogation is described and characterized. Based upon the modification of a commercially available current loop-actuated design, this apparatus produces conductance-limited pulses of ⩽50 μs duration at repetition rates of up to 20 pps. The short temporal length of the emerging gas pulse greatly alleviates the demands imposed upon vacuum pumping systems and enables the use of higher stagnation pressures in the expansion process.

Pseudorotation in Water Trimer Isotopomers Using Terahertz Laser Spectroscopy

We report the first observation of five water trimer isotopomers using terahertz laser spectroscopy coupled with a pulsed slit jet expansion technique. A single c-type vibration−rotation-tunneling (VRT) band has been observed for each isotopomer between 40 and 50 cm-1. By considering all the experimental data, including results from isotopic substitution experiments, analyses of bifurcation tunneling splittings, and rotational analyses of the VRT bands, it has been possible to determine unambiguously the isotopic composition and structure of each isotopomer. We have also extended the measurements of the 41.1 cm-1 band in (D2O)3, first reported by Suzuki and Blake [Suzuki, S.; Blake, G. A. Chem. Phys. Lett. 1994, 229, 499]. All six VRT bands have been assigned to pseudorotational transitions, which principally involve flipping of the “free” hydrogen or deuterium atoms in the trimer from above to below the plane of the OOO ring. Interestingly, four of the six bands have been assigned to the same transition. The four trimers responsible for these bands, designated d6, d5a, d4a, and d3a, form a series in which the three deuterium atoms in the ring are sequentially substituted by hydrogen atoms. The corresponding experimental band origins for d6 to d3a show a 1−2 cm-1 blue shift upon each substitution. Existing pseudorotation models are unable to satisfactorily account for this result. We propose that, if the hydrogen or deuterium atoms within the ring are allowed to participate in the flipping motion, this band origin shift can be explained.

Slit jet infrared absorption spectroscopy of N2-HCl complexes

The vibration–rotation spectra of the weakly bound complexes N2-HCl have been studied in the 3·5 μm region of the υ1 fundamental band correlated with the HCl stretch, using a pulsed slit jet expansion and a tunable diode laser spectrometer. For the first time the υ1 band for the N2-H37 Cl isotope has been analysed and the rotational constants determined. The υ1 band was observed previously for the N2– H35 Cl isotope using a cooled long path cell and a Fourier transform spectrometer. This band has been re-examined to confirm and extend this analysis, with new lines corresponding to low J values observed and assigned. The rotational molecular constants are in agreement with those determined already, but for the band centre the value found was slightly greater than those obtained previously. For both isotopes, the observed spectra confirm the quasilinear geometry of the complex.

The application of pulsed slit-jet expansion sources in sub-Doppler degenerate four-wave mixing spectroscopy

Sub-Doppler degenerate four-wave mixing (DFWM) spectroscopy has been performed on carbon disulfide (CS 2) molecules entrained in a pulsed slit-jet expansion of modest dimensions (i.e. ≈ 1 cm slit length). Data obtained for the 10V band in the V 1 B 2( 1 Δ u ) ← X ̃ 1 Σ g + ( π ∗ ← π) absorption system (≈ 323.7 nm) demonstrate significant enhancements in sensitivity and resolution over that obtained through use of a conventional free-jet source based upon a circular nozzle orifice. These observations are quantified and interpreted in terms of the increase in effective sample interaction length that accompanies collapse of the transverse velocity distribution in a slit-jet expansion.