Christopher M. Lovejoy

High sensitivity, high‐resolution IR laser spectroscopy in slit supersonic jets: Application to N2HF ν1 and ν5+ν1−ν5

A difference frequency IR spectrometer is combined with a slit supersonic expansion for high‐resolution (≤50 MHz FWHM) direct absorption investigations of jet‐cooled species. The 1.25 cm long nozzle provides a long path length and high densities suitable for synthesis and observation of van der Waals clusters, with a gradual spatial temperature gradient that permits experimental control of low frequency vibrational populations. Due to collisional quenching of velocity distributions, absorption linewidths are reduced and peak absorbance increased five‐ to sevenfold compared to pinhole expansions. Minimum detectable concentrations of HF containing complexes are 2×109 molecules/cm3/quantum state in a 2.5 cm path length. The combination of high sensitivity, sub‐Doppler resolution, long path lengths, and temperature control make direct absorption in slit nozzle expansions a powerful and general technique for high‐resolution study of jet‐cooled species. The spectometer is used to obtain the near‐infrared spectr...

Slit pulsed valve for generation of long-path-length supersonic expansions

We describe a valve for production of jet‐cooled species in a pulsed, long‐path‐length (1.2‐cm) supersonic expansion. The valve produces 150–600‐μs‐duration pulses at repetition rates up to 60 Hz from a nozzle with variable slit width, and is suitable for use with corrosive gases and vapors.

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] ...

Hindered internal rotation in jet cooled H2HF complexes

The vibration–rotation spectrum of the HF stretch mode in ortho‐H2HF complexes has been obtained via infrared laser direct absorption detection in a slit supersonic jet expansion. The spectrum resembles a Ka =1←1 parallel band of a prolate near‐symmetric top and can be reasonably well fit with a Watson A‐type Hamiltonian; however, no rigid molecular structure can reproduce the observed Ka splittings without invoking unphysically large changes in the constituent bond lengths upon complexation. The splittings are more correctly analyzed in terms of a j=1 hindered H2 rotor in an anisotropic potential, with a minimum energy T‐shaped geometry. Matrix calculations determine barriers to H2 rotation between 120 and 170 cm−1 that depend systematically both on vibrational and rotational state in the complex. These data are consistent with a strong increase in potential anisotropy with decreasing intermolecular separation, with both upper and lower vibrational states close to the dissociation limit. No evidence for ...

Intramolecular dynamics of van der Waals molecules: An extended infrared study of ArHF

The near‐infrared spectrum of ArHF prepared in a slit supersonic expansion is recorded with a difference frequency infrared laser spectrometer. By virtue of the high sensitivity of the technique, and the lack of appreciable spectral congestion at the 10 K jet temperature, we observe 9 of the 11 vibrational states with energies below the Ar+HF(v=1, j=0) dissociation limit. These include (1000), the lowest bound HF (v=1) state, the singly, doubly, and quadruply van der Waals stretch excited states (1001) (1002), and (1004), both the Σ bend (1200) and Π bend (111e,f 0), and the multiply excited, Π bend plus van der Waals stretch (111e,f 1). Two Ar+HF(v=0) states, (0000) and (0001), are also characterized. This spectroscopic information is quite sensitive to the Ar+HF potential energy surface away from the equilibrium configuration, and thus provides a rigorous test of trial potential energy surfaces. Excellent agreement is obtained between experiment and the predictions of a recently reported Ar+HF(v=1) pote...

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...

Mode specific internal and direct rotational predissociation in HeHF, HeDF, and HeHCl: van der Waals complexes in the weak binding limit

The near‐infrared vibration–rotation spectra of the weakly bound complexes HeHF, HeDF, and HeHCl are observed in a slit supersonic expansion. The spectra correspond to simultaneous excitation of vibration and internal rotation of the H(D)X subunit within the complex. The HeHF and HeDF P/R branch transitions show J‐dependent excess linewidths, which are attributed to rapid predissociation of the excited states from intramolecular rotation–translation energy transfer. The corresponding P/R branch transitions in HeHCl are not observed despite good S/N on the Q branch, suggesting even more rapid predissociation for the upper state of this complex. The Q branch transitions for all three complexes abruptly terminate at low J, yielding lower limits to the number of bound rotational states and good estimates of the dissociation energies D0=7.1±0.1 cm−1 for HeHF and HeDF, and 10.1±1.2 cm−1 for HeHCl. In addition to isotropic intermolecular potentials, the HeHF/HeDF data yield considerable information on the potent...

The near-infrared spectrum of ONNHF—direct evidence for geometric isomerism in a hydrogen bonded complex

The near‐IR spectrum and equilibrium structure of a novel hydrogen bonded complex between nitrous oxide and hydrogen fluoride is described. In contrast to a previously reported structure, in which the HF bonds to the oxygen end of NNO, the present structure has the HF bonded to the nitrogen end of NNO. The structure is unambiguously confirmed by isotopic substitution. The identification of the two stable, spectroscopically distinct structures represents the first demonstration of geometric isomerism in a hydrogen‐bonded complex.

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...

The infrared spectra of nitrous oxide–HF isomers

Two spectroscopically distinct isomers of a hydrogen bonded complex between nitrous oxide and hydrogen fluoride are observed by direct infrared laser absorption detection in a slit supersonic expansion. The linear isomer FH–NNO contains a relatively rigid hydrogen bond to the nitrogen end of NNO. The bent isomer NNO–HF has a stronger hydrogen bond to the oxygen end of NNO, but this bond is characterized by a softer bending potential and thus the complex exhibits evidence of large amplitude bending motion. Rapid vibrational predissociation, as determined from the homogeneous broadening of the rovibrational absorption structure, is evidenced in both isomers. The linear isomer exhibits predissociation lifetimes which show structure as a function of the upper J’ rotational level, including narrow resonances which suggest excitation of NNO fragment vibrational modes.