Michael D. Schuder

High resolution IR laser spectroscopy of van der Waals complexes in slit supersonic jets: Observation and analysis of ν1, ν1+ν2, and ν1+2ν3 in ArHF

IR spectra of jet cooled ArHF are obtained via direct absorption of a high resolution tunable difference frequency laser in a 2.54 cm path length, slit supersonic pulsed expansion at <10 K. Detection limits of 2×109 molecules/cm3/quantum state permit observation of the high frequency ν1 fundamental stretch (1000) ← (0000), the ν1+ν2 van der Waals bend plus stretch combination band (1110) ← (0000), as well as transitions to the (1002) triply vibrationally excited state that are weakly allowed via Coriolis interactions with the Π+ component of the (1110) manifold. The ground state (0000) molecular constants are in excellent agreement with previous microwave data. From the changes in rotational and centrifugal distortion constants, the vibrationally averaged van der Waals well depth is estimated to increase (+15%) with ν1 excitation, but decrease dramatically (−42%) upon subsequent excitation of the l=1 ν2 bend. L‐doubling in the ν1+ν2 (1110) perpendicular bending state is large and negative [−69.8(18) MHz] ...

High resolution, jet‐cooled infrared spectroscopy of (HCl)2: Analysis of ν1 and ν2 HCl stretching fundamentals, interconversion tunneling, and mode‐specific predissociation lifetimes

An extensive series of near‐infrared absorption spectra are recorded for jet‐cooled (6–14 K) hydrogen chloride dimer (HCl)2. Both ΔKa=0 and ΔKa=±1 bands are observed for both the free (ν1) and bonded (ν2) HCl stretches; all three chlorine isotopomers (H 35Cl–H 35Cl, H 35Cl–H 37Cl, and H 37Cl–H 37Cl) are observed and analyzed for K‘a ≤ 2. The slit jet spectrum extends significantly the previous cooled cell infrared study of this complex and provides a measure of tunneling splittings for Ka=0 and 1 for each of the HCl ground (v=0) and excited (v=1) states. Mode specific vibrational predissociation is observed via analysis of the absorption line shapes, with Lorentzian contributions to the line profiles of Δν1≲1.6 MHz and Δν2=5.1±1.2 (2σ) MHz full width at half‐maximum for ν1 and ν2 excitation, respectively. Stronger coupling in (HCl)2 of the bonded (ν2) vs free (ν1) HCl vibration to the dissociation coordinate is consistent with the comparable trends observed in other hydrogen bonded dimers. Quantum mechani...

Direct IR laser absorption spectroscopy of jet-cooled CO2HF complexes: Analysis of the ν1 HF stretch and a surprisingly low frequency ν6 intermolecular CO2 bend

High sensitivity, tunable laser direct absorption methods are exploited to obtain high resolution IR spectra (Δν≲0.001 cm−1) of weakly bound CO2HF complexes in a pulsed supersonic slit jet expansion. Transitions from the ground vibrational state corresponding to a single quantum excitation of the ν1 HF stretch are observed and analyzed with a semirigid linear molecule Hamiltonian. The observed increase in both B (+1.75%) and D (+55%) upon ν1 excitation is inconsistent with the commonly used diatomic approximation, and is not possible to rationalize for a nearly linear upper state geometry with small amplitude zero point motion of the intermolecular CO2 bend coordinate. We consider an alternative centrifugal straightening mechanism which predicts large centrifugal distortion effects due to end over end rotation of a complex with a nonlinear vibrationally averaged geometry in a weak bending potential. In support of this interpretation, hot band spectra are observed arising from bend excited complexes signif...

Sub-doppler infrared absorption spectroscopy of Ar-HF [(1000) ← (0000)] in a linear supersonic jet

Abstract Ultra-sensitive tunable difference frequency IR absorption spectroscopy in a slit supersonic jet has been used to observe sub-Doppler spectra of Ar-HF in the (1000) HF stretch and (1110) HF stretch plus van der Waals bend modes. Linewidths yield a lower limit of 3 × 10−9 s for the predissociation lifetime in the vibrationally metastable upper state. The sensitivity of these direct absorption methods (≲ 2 × 109 molecules/cm3 per quantum state), in conjunction with the wide tunability of the difference frequency laser (2.2–4.2 μm) permit high-resolution studies of a large class of van der Waals complexes.

Symmetry breaking in HCl and DCl dimers: a direct near-infrared measurement of interconversion tunneling rates

The interconversion tunneling frequencies for (HCl)2 and (DCl)2 are obtained from near‐infrared absorption spectra of the H(D)Cl stretching transitions, to spectroscopic precision for the mixed 35Cl–37Cl dimers. A phenomenological model of the interconversion process explains several experimental observations, and provides good estimates of the splittings expected for the 35Cl–35Cl and 37Cl–37Cl species.

Slit-jet near-infrared diode laser spectroscopy of (DCl)2: ν1, ν2 DCl stretching fundamentals, tunneling dynamics, and the influence of large amplitude ‘‘geared’’ intermolecular rotation

The first high resolution spectra of (DCl)2 are reported using direct IR laser absorption spectroscopy in a slit supersonic expansion. The spectral data are analyzed to obtain vibrational frequencies, rotational constants, and tunneling (interconversion) level splittings for isotopically symmetric (D35Cl)2 and (D37Cl)2, and mixed D35Cl–D37Cl dimers. Six dimer absorption bands are observed and analyzed for both (D35Cl)2 and D35Cl–D37Cl. These include two perpendicular Ka=1←0, v1=1←0 (i.e., ‘‘free’’ DCl stretch) bands, one each originating from the lower (+) and the upper (−) tunneling sublevels in the ground vibrational state. Four parallel v2=1←0 (i.e., ‘‘bound’’ DCl stretch) bands are also observed, one for each of the Ka=0←0 and Ka=1←1 subbands originating from both the lower (+) and upper (−) tunneling components.In addition, two bands are observed only for the isotopically mixed dimer (i.e., complexes from D35Cl and D37Cl), which acquire oscillator strength by virtue of the breaking of inversion symme...

High resolution near infrared spectroscopy of HCl–DCl and DCl–HCl: Relative binding energies, isomer interconversion rates, and mode specific vibrational predissociation

Both D‐ and H‐bonded isomers of the mixed dimers formed between HCl and DCl are investigated via high resolution infrared difference frequency and diode laser spectroscopy in the 2885 and 2064 cm−1 regions. From an analysis of the relative integrated absorption intensities, the D‐bonded complex (i.e., HCl–DCl) is determined to be more stable by 16±4 cm−1 than the H‐bonded (i.e., DCl–HCl) species. All four chlorine isotopic combinations of the lower energy (HCl–DCl) complex are probed via excitation of both HCl (vHClacc=1←0) and DCl (vDCldon=1←0) stretches. Additionally, two chlorine isotopomers of the higher energy (DCl–HCl) complex are investigated through HCl excitation. Compared to the facile tunneling observed in both (HCl)2 or (DCl)2 complexes, these mixed dimers exhibit more rigid behavior characteristic of two distinct isomeric species. However, the relatively small energy difference (16±4 cm−1) between the two isomers still allows the wave functions for both species to sample both the HCl–DCl and ...

Investigation of internal rotor dynamics of NeDCl and ArDCl via infrared absorption spectroscopy

The van der Waals complexes, NeDCl and ArDCl, are produced in a slit jet supersonic expansion and observed via direct absorption of tunable mid‐infrared Pb‐salt diode laser radiation. For the NeDCl complex, the DCl stretch fundamental [ν0=2091.3717(4) cm−1 ] and the DCl Σ and Πe, f bend combination bands [ν0=2099.5760(4) and 2104.9465(4) cm−1, respectively] are observed. The DCl stretch fundamental and Πe, f combination band are observed for ArDCl at 2089.4180(2) and 2117.4443(3) cm−1, respectively. The relative fundamental vs bend combination band intensity distributions are very different for the two complexes. The ArDCl fundamental to Π bend combination band intensity ratio is 4:1, whereas for NeDCl the corresponding ratio is 1:8. This anomalous intensity pattern for NeDCl and the proximity of the bend combination bands to the DCl R(0) line indicate that the DCl diatomic is exhibiting nearly free rotation within this complex, compared to more restricted librational motion of DCl in ArDCl. Strong Coriol...

Large-amplitude motion in highly quantum clusters: high-resolution infrared absorption studies of jet-cooled H2–HCl and H2–DCl

Abstract High-resolution infrared spectroscopy is used to interrogate a series of inter and intramolecular vibrational quantum states in jet-cooled ortho and para H 2 –HCl and H 2 –DCl complexes, which as a result of weak binding and large zero-point effects provide a novel dynamical window into large-amplitude motion in highly quantum mechanical clusters. The fundamental v HCl =1←0 stretch bands of H 2 –HCl/DCl are observed and elucidate dramatic differences in the vibrationally averaged intermolecular alignment of the H 2 subunit, i.e., T-shaped vs. more nearly isotropic for the lowest ortho (Π) and para (Σ) nuclear spin states, respectively. The two internal-rotor states correlating with H 2 ( j =1) in the o -H 2 –HCl complex are observed via fundamental and combination band excitation built on v HCl =1←0. The 8.5 cm −1 internal-rotor splitting between the ground (Π) and excited (Σ) H 2 alignments confirms the T-shaped minimum energy configuration for the intermolecular potential, with the HCl proton donating into the H 2 subunit. At even higher energies for o -H 2 –HCl, a rich but highly perturbed spectrum of combination band transitions is observed due to the strongly Coriolis coupled manifold ((2 j H 2 +1)(2 j HCl +1)=9) of levels correlating with H 2 ( j =1) and HCl( j =1) subunits. Rotational predissociation broadening accompanied by an abrupt cut off in these combination band spectra is observed and used to estimate a dissociation energy window of D 0 =45±2 and 47±2 cm −1 for the v HCl =0 and 1 intramolecular HCl stretching states, respectively, of the o -H 2 –HCl complex.

Probing potential surfaces for hydrogen bonding: Near-infrared combination band spectroscopy of van der Waals stretch (ν4) and geared bend (ν5) vibrations in (HCl)2

High resolution infrared spectra of the two “low” frequency intermolecular modes—van der Waals stretch (ν4) and geared bend (ν5) of (HCl)2—have been characterized in HCl-stretch excited states using a slit jet spectrometer. In a high resolution high sensitivity search covering the range between 2880 and 3070 cm−1 four (HCl)2 combination bands associated with in-plane vibrations ν4 and ν5 have been observed. The vibrational assignment of these bands is based on comparison between observed intermolecular mode energies and predictions from recent six-dimensional (6D) quantum mechanical (QM) calculations [Y. Qiu, J. Z. H. Zhang, and Z. Bacic, J. Chem. Phys. 108, 4804 (1998)], though additional confirmation is provided by ancillary spectroscopic information such as rotational constants, predissociation linewidths, and 35Cl/37Cl isotopic band shifts. The experimentally observed intermolecular energies agree with theoretical predictions to ≈2–4 cm−1 out of 60–90 cm−1, suggesting that the 6D potential energy surf...