Slit Jet Expansion Research Articles

Sub-Doppler infrared spectroscopy and formation dynamics of triacetylene in a slit supersonic expansion.

Infrared spectroscopy and formation dynamics of triacetylene are investigated in a slit jet supersonic discharge and probed with sub-Doppler resolution (≈60 MHz) on the fundamental antisymmetric CH stretch mode (ν5). The triacetylene is generated in the throat of the discharge by sequential attack of ethynyl radical with acetyelene and diacetylene: (i) HCCH → HCC + H, (ii) HCC + HCCH → HCCCCH + H, (iii) HCC + HCCCCH → HCCCCCCH + H, cooled rapidly in the slit expansion to 15 K, and probed by near shot-noise-limited absorption sensitivity with a tunable difference-frequency infrared laser. The combination of jet cooled temperatures (Trot = 15 K) and low spectral congestion permits (i) analysis of rotationally avoided crossings in the ν5 band ascribed to Coriolis interactions, as well as (ii) first detection of ν5 Π-Π hot band progressions built on the ν12 sym CC bend and definitively assigned via state-of-the-art ab initio vibration-rotation interaction parameters (αi), which make for interesting comparison with recent spectroscopic studies of Doney et al. [J. Mol. Spectrosc. 316, 54 (2015)]. The combined data provide direct evidence for significantly non-equilibrium populations in the CC bending manifold, dynamically consistent with a strongly bent radical intermediate and transition states for forming triacetylene product. The presence of intense triacetylene signals under cold, low density slit jet conditions provides support for (i) barrierless addition of HCC with HCCCCH and (ii) a high quantum yield for HCCCCCCH formation. Complete basis set calculations for energetics [CCSD(T)-f12/VnZ-f12, n = 2,3] and frequencies [CCSD(T)-f12/VdZ-f12] are presented for both radical intermediate and transition state species, predicting collision stabilization in the slit jet expansion to be competitive with unimolecular decomposition with increasing polyyne chain length.

The infrared spectrum of the Ne-C2D2 complex.

Infrared spectra of Ne-C2D2 are observed in the region of the ν3 fundamental band (asymmetric C-D stretch, ≈2440 cm(-1)) using a tunable optical parametric oscillator to probe a pulsed supersonic slit jet expansion from a cooled nozzle. Like helium-acetylene, this system lies close to the free rotor limit, making analysis tricky because stronger transitions tend to pile up close to monomer (C2D2) rotation-vibration transitions. Assignments are aided by predicted rotational energies calculated from a published ab initio intermolecular potential energy surface. The analysis extends up to the j = 3←2 band, where j labels C2D2 rotation within the dimer, and is much more complete than the limited infrared assignments previously reported for Ne-C2H2 and Ne-C2HD. Two previous microwave transitions within the j = 1 state of Ne-C2D2 are reassigned. Coriolis model fits to the theoretical levels and to the spectrum are compared. Since the variations observed as a function of C2D2 vibrational excitation are comparable to those noted between theory and experiment, it is evident that more detailed testing of theory will require vibrational averaging over the acetylene intramolecular modes.

Sub-Doppler infrared spectroscopy of propargyl radical (H2CCCH) in a slit supersonic expansion.

The acetylenic CH stretch mode (ν1) of propargyl (H2CCCH) radical has been studied at sub-Doppler resolution (∼60 MHz) via infrared laser absorption spectroscopy in a supersonic slit-jet discharge expansion, where low rotational temperatures (Trot = 13.5(4) K) and lack of spectral congestion permit improved determination of band origin and rotational constants for the excited state. For the lowest J states primarily populated in the slit jet cooled expansion, fine structure due to the unpaired electron spin is resolved completely, which permits accurate analysis of electron spin-rotation interactions in the vibrationally excited states (εaa = - 518.1(1.8), εbb = - 13.0(3), εcc = - 1.8(3) MHz). In addition, hyperfine broadening in substantial excess of the sub-Doppler experimental linewidths is observed due to nuclear spin-electron spin contributions at the methylenic (-CH2) and acetylenic (-CH) positions, which permits detailed modeling of the fine/hyperfine structure line contours. The results are consistent with a delocalized radical spin density extending over both methylenic and acetylenic C atoms, in excellent agreement with simple resonance structures as well as ab initio theoretical calculations.

Spectroscopy and Dynamics of Jet-Cooled Polyynes in a Slit Supersonic Discharge: Sub-Doppler Infrared Studies of Diacetylene HCCCCH.

Fundamental, bending (ν6, ν7, ν8, ν9), and CC-stretch (ν2, ν3) hot band spectra in the antisymmetric CH stretch (ν4) region near 3330 cm(-1) have been observed and analyzed for jet cooled diacetylene (HC≡C-C≡CH) under sub-Doppler conditions. Diacetylene is generated in situ in the throat of a pulsed supersonic slit expansion by discharge dissociation of acetylene to form ethynyl (C≡CH) + H, followed by radical attack (HC≡CH + C≡C-H) to form HC≡C-C≡CH + H. The combination of (i) sub-Doppler line widths and (ii) absence of spectral congestion permits rotational structure and Coriolis interactions in the ν4 CH stretch fundamental to be observed and analyzed with improved precision. Of particular dynamical interest, the spectra reveal diacteylene formation in highly excited internal vibrational states. Specifically, multiple Π ← Π and Δ ← Δ hot bands built on the ν4 CH stretch fundamental are observed, due to doubly degenerate bending vibrations [cis C≡C-H bend (ν6), trans C-C≡C bend (ν7), trans C≡C-H bend (ν8) and cis C-C≡C bend (ν9)], as well as a heretofore unobserved Σ ← Σ band assigned to excitation of ν2 or 2ν3 CC stretch. Boltzmann analysis yields populations consistent with universally cold rotations (Trot ≈ 15 ± 5 K) and yet superthermal vibrations (Tvib ≈ 85-430 K), the latter of which is quite anomalous for the high collision densities in a slit jet expansion. In order to elucidate the physical mechanism for this excess vibrational excitation, high level ab initio CCSD(T) calculations have been pursued with explicitly correlated basis sets (VnZ-f12; n = 2,3) and extrapolated to the complete basis set (CBS) limit using MOLPRO quantum chemistry software. The results suggest that the extensive hot band structure observed arises from (i) highly exothermic CCH + HCCH addition to yield a strongly bent HCCHCCH radical intermediate (ΔH = -62.6 kcal/mol), followed by (ii) rapid fragmentation over a submerged transition state barrier (ΔH = -18.9 kcal/mol) to form vibrationally hot diacetylene + H products (ΔH = -25.6 kcal/mol), and consistent with crossed molecular beam studies by Kaiser et al. [Phys. Chem. Chem. Phys. 2002, 4, 2950.] Finally, RRKM fragmentation rates for this complex are calculated, which exceed collision frequencies in the slit jet expansion and suggest near unity quantum efficiency for diacetylene formation.

The infrared spectrum of the He-C2D2 complex.

Spectra of the helium-acetylene complex are elusive because this weakly bound system lies close to the free rotor limit. Previously, limited assignments of He-C2D2 transitions in the R(0) region of the ν3 fundamental band (≈2440 cm(-1)) were published. Here, new He-C2D2 infrared spectra of this band are obtained using a tunable optical parametric oscillator laser source to probe a pulsed supersonic slit jet expansion from a cooled nozzle, and the analysis is extended to the weaker and more difficult P(1) and R(1) regions. A term value approach is used to obtain a consistent set of "experimental" energy levels. These are compared directly with calculations using two recently reported ab initio intermolecular potential energy surfaces, which exhibit small but significant differences. Rovibrational energies for the He-C2H2 complex are also calculated using both surfaces. A Coriolis model, useful for predicting spectral intensities, is used to interpret the energy level patterns, and a comparison with the isoelectronic complex He-CO is made.

Observation of mixed acetylene – Nitrous oxide trimers: Infrared spectra of C2H2–(N2O)2 and (C2H2)2–N2O

Infrared spectra of the lowest energy isomers of C2H2–(N2O)2 and (C2H2)2–N2O were observed in the region of the ν1 fundamental band of the N2O monomer (∼2224cm−1) using a tunable diode laser and/or a CW quantum cascade laser to probe a pulsed supersonic slit jet expansion. One infrared band was measured for each trimer. The band for C2H2–(N2O)2 corresponds to the out-of-phase vibrations of the pair of equivalent N2O monomers. It is blue shifted by about 10cm−1with respect to the free N2O monomer. The band for (C2H2)2–N2O is slightly less blue shifted (centered at 2232.81cm−1). It can be simulated as a predominately a-type band. In addition to the normal isotopologues,the corresponding bands for C2D2–(N2O)2 and (C2D2)2–N2Owere also observed. The structures of these trimers are similar to those of the lowest energy isomers of the analogous OCS – acetylene trimers reported previously, that is, a twisted barrel with C2 symmetry for C2H2–(N2O)2 and a distorted T-shaped acetylene dimer with a coplanar N2O beside the stem of the T for (C2H2)2–N2O. Here, we present our observation and experimental results, which agree well with calculations based on distributed multipole pair potentials.

Fundamental and combination bands of CO2–C2H2 and CO2–C2D2 in the mid-infrared region

Spectra of the weakly bound CO2–C2H2 and CO2–C2D2 complexes are observed in the regions of CO2 ν3 (≈ 2349 cm−1) and C2D2 ν3 (≈ 2440 cm−1) fundamental vibrations, using an infrared optical parametric oscillator to probe a pulsed supersonic slit-jet expansion. Five bands are measured and analysed: the fundamental asymmetric stretch of the C2D2 component, two combination bands involving the out-of-plane torsional vibrations (C2D2 ν3 + torsion and CO2 ν3 + torsion) for CO2–C2D2, and two combination bands involving an intermolecular in-plane bending vibration for CO2–C2H2 and CO2–C2D2. The resulting intermolecular frequencies are 61.408(1), 54.5(5), 39.9(5), and 39.961(1) cm−1 for CO2–C2H2 and CO2–C2D2 in-plane vibrations, and CO2–C2D2 out-of-plane torsional vibrations in CO2 and C2D2 regions, respectively. This is the first experimental determination of these intermolecular vibrational frequencies.

Infrared spectra and tunneling dynamics of the N2–D2O and OC–D2O complexes in the v2 bend region of D2O

The rovibrational spectra of the N2-D2O and OC-D2O complexes in the v2 bend region of D2O have been measured in a supersonic slit jet expansion using a rapid-scan tunable diode laser spectrometer. Both a-type and b-type transitions were observed for these two complexes. All transitions are doubled, due to the heavy water tunneling within the complexes. Assuming the tunneling splittings are the same in K(a) = 0 and K(a) = 1, the band origins, all three rotational and several distortion constants of each tunneling state were determined for N2-D2O in the ground and excited vibrational states, and for OC-D2O in the excited vibrational state, respectively. The averaged band origin of OC-D2O is blueshifted by 2.241 cm(-1) from that of the v2 band of the D2O monomer, compared with 1.247 cm(-1) for N2-D2O. The tunneling splitting of N2-D2O in the ground state is 0.16359(28) cm(-1), which is about five times that of OC-D2O. The tunneling splittings decrease by about 26% for N2-D2O and 23% for OC-D2O, respectively, upon excitation of the D2O bending vibration, indicating an increase of the tunneling barrier in the excited vibrational state. The tunneling splittings are found to have a strong dependence on intramolecular vibrational excitation as well as a weak dependence on quantum number K(a).

Sub-Doppler Spectroscopy of the trans-HOCO Radical in the OH Stretching Mode

Rovibrational spectroscopy of the fundamental OH stretching mode of the trans-HOCO radical has been studied via sub-Doppler high-resolution infrared laser absorption in a discharge slit-jet expansion. The trans-HOCO radical is formed by discharge dissociation of H2O to form OH, which then combines with CO and cools in the Ne expansion to a rotational temperature of 13.0(6) K. Rigorous assignment of both a-type and b-type spectral transitions is made possible by two-line combination differences from microwave studies, with full rovibrational analysis of the spectrum based on a Watson asymmetric top Hamiltonian. Additionally, fine structure splittings of each line due to electron spin are completely resolved, thus permitting all three ε(aa), ε(bb), ε(cc) spin-rotation constants to be experimentally determined in the vibrationally excited state. Furthermore, as both a- and b-type transitions for trans-HOCO are observed for the first time, the ratio of transition dipole moment projections along the a and b principal axes is determined to be μ(a)/μ(b) = 1.78(5), which is in close agreement with density functional quantum theoretical predictions (B3LYP/6-311++g(3df,3pd), μ(a)/μ(b) = 1.85). Finally, we note the energetic possibility in the excited OH stretch state for predissociation dynamics (i.e., trans-HOCO → H + CO2), with the present sub-Doppler line widths providing a rigorous upper limit of >2.7 ns for the predissociation lifetime.

Infrared spectra of the Ne[sbnd]C2D4 and Ar[sbnd]C2D4 complexes

Spectra of NeC2D4 and ArC2D4 are studied in the ν11 fundamental band region of C2D4 (≈2200cm−1) using a tuneable quantum cascade laser to probe a pulsed supersonic slit jet expansion. The spectra are those of perpendicular bands of near-prolate asymmetric rotors, but with a doubling of transitions due to tunneling. These tunneling splittings are due to hindered internal rotation of C2D4 around its CC axis, and are more prominent for NeC2D4 than for ArC2D4. The analysis yields the first direct measurement of the A rotational constants for these complexes, and shows that the vibrational shifts are very small (<0.3cm−1). Attempts to observe the spectrum of HeC2D4 were not successful.

Infrared spectrum of the CS2 tetramer: Observation of a structure with D2d symmetry

Infrared spectra of carbon disulfide clusters are studied in the region of the CS2 ν3 fundamental band (≈1535cm−1), using a tuneable diode laser to probe a pulsed supersonic slit jet expansion. A symmetric rotor parallel band at 1551.438cm−1 is assigned to CS2 tetramer. The likely structure is a staggered and tilted barrel with D2d symmetry, similar to the previously observed oblate tetramer of nitrous oxide, and in reasonable agreement with that calculated from an empirical CS2 intermolecular potential function. This is the first high-resolution spectroscopic detection of CS2 tetramer, and the calculations suggest that there could be other low energy isomers of (CS2)4 which are not yet observed.

Fundamental and Torsional Combination Bands of Two Isomers of the OCS–CO2 Complex in the CO2 ν3 Region

Spectra of two isomers of the weakly bound complex OCS-CO2 are observed in the region of the CO2 ν3 fundamental vibration (∼2349 cm(-1)), using an infrared tunable diode laser to probe a pulsed supersonic slit-jet expansion. Two bands are measured and analyzed for each isomer, the fundamental asymmetric stretch of the CO2 component and a combination band involving this fundamental plus the intermolecular out-of-plane torsional mode. For one isomer, the corresponding torsional combination band is also detected in the OCS ν1 stretching region (∼2060 cm(-1)). The resulting torsional frequencies are found to be 18.8 and 15.9 cm(-1) for isomers a and b of OCS-CO2, respectively. This may be the first time that such a combination band is observed for a higher-energy isomer of a weakly bound complex.

Three-level depletion by cavity ringdown absorption spectroscopy: proof of concept

Theoretical quantitative considerations as well as experimental data are presented based on absorption population depletion coupled with cavity ringdown spectroscopy. The absorbing number densities inside the cavity are determined by numerical integration of the coupled rate equations. The number of photons involved in absorption, cavity losses due to mirror reflectivity and stimulated emission are taken into account. The principle is to monitor a first transition by cavity ringdown spectroscopy while a second transition, with a state in common, is resonantly excited by the decaying radiation of different frequency also trapped inside the optical cavity. A numerical example is given for atomic lines of neon and the measurements carried out in a supersonic slit-jet expansion discharge demonstrate the feasibility of the technique. The technique is also proven to work with two resonant transitions of C2. Translational velocity of the jet modifying the rate equations is included in the model.

New infrared bands of nonpolar OCS dimer and experimental frequencies for two intermolecular modes

Spectra of the nonpolar carbonyl sulfide dimer in the region of the OCS ν(1) fundamental band were observed in a slit-jet supersonic expansion. The jet was probed using radiation from a tunable diode laser employed in a rapid-scan signal averaging mode. Six new bands were observed and analyzed, all of which originate from the dimer ground vibrational state. Three were vibrational fundamentals involving the ((18)OCS)(2) and (16)OCS-(18)OCS isotopologues. They enabled an estimate to be made of the frequency of the infrared-forbidden mode corresponding to in-phase vibration of the OCS monomers in the dimer, a value needed to obtain an intermolecular vibrational frequency from one of the observed combination bands. A relatively weak b-type dimer band centered at 2103.105 cm(-1) was assigned to the OCS 4ν(2) (l = 0) bending overtone. Combination bands were observed involving the geared bend and van der Waals stretch intermolecular modes. The resulting experimental frequencies of 37.5(20) cm(-1) for the bend and 42.9727(1) cm(-1) for the stretch are in good agreement with a recent high level theoretical calculation.

Towards an understanding of the helium–acetylene van der Waals complex

The weakly bound complex He–C2D2 is studied in the ν 3 fundamental band of C2D2 (≈2440 cm−1) using a tuneable infrared diode laser to probe a pulsed supersonic slit jet expansion. This is the first published spectrum for helium–acetylene. Transitions observed in the region of the C2D2 R(0) line are assigned with the help of theoretical results based on an ab initio intermolecular potential, and fitted using a simple Coriolis model. The results indicate that the complex is rather close to the free rotor limit, helping to explain the absence of previous data. Scaled parameters from the model are used to predict a spectrum for He–C2H2.

New spectroscopic results on acetylene dimers and trimers

Spectra of acetylene dimers and trimers containing one or more C2D2 monomer are studied in the ν3 fundamental band region of C2D2 (≈2440 cm−1) using a tuneable infrared diode laser to probe a pulsed supersonic slit jet expansion. Four new subbands are observed in the perpendicular band of (C2D2)2, and this enables the first direct determination of the A rotational constant for an acetylene dimer. The value found for A is significantly larger than the previous indirect value based on microwave spectra. The dimer parallel band is observed and found to be highly perturbed, and observations are extended for the mixed dimer C2D2 – C2H2. The trimers (C2D2)3, (C2D2)2 – C2H2, and C2D2 – (C2H2)2 are observed spectroscopically for the first time. Establishment of a precise band origin is difficult for (C2D2)3 because of the inherent nature of the spectrum, but this is possible for two out of three of the bands of the mixed trimers.

Communication: The highest frequency hydrogen bond vibration and an experimental value for the dissociation energy of formic acid dimer

The highest frequency hydrogen bond fundamental of formic acid dimer, ν(24) (B(u)), is experimentally located at 264 cm(-1). FTIR spectra of this in-plane bending mode of (HCOOH)(2) and band centers of its symmetric D isotopologues (isotopomers) recorded in a supersonic slit jet expansion are presented. Comparison to earlier studies at room temperature reveals the large influence of thermal excitation on the band maximum. Together with three B(u) combination states involving hydrogen bond fundamentals and with recent progress for the Raman-active modes, this brings into reach an accurate statistical thermodynamics treatment of the dimerization process up to room temperature. We obtain D(0) = 59.5(5) kJ/mol as the best experimental estimate for the dimer dissociation energy at 0 K. Further improvements have to wait for a more consistent determination of the room temperature equilibrium constant.

CMM-RS Potential for Characterization of the Properties of the Halogen-Bonded OC–Cl2 Complex, and a Comparison with Hydrogen-Bonded OC–HCl

Transitions associated with the vibrations ν₁, ν₁ + ν(b)¹, ν₁ + ν₅¹, and ν₁ + ν₅¹ - ν₅¹ of the complex OC···Cl₂ have been rovibrationally analyzed for several isotopologues involving isotopic substitutions in Cl₂. Spectra were recorded using a recently constructed near-infrared (4.34 to 4.56 μm), quantum-cascade laser spectrometer with cw supersonic slit jet expansion. Spectral analysis allowed precise determination of the ν₅¹ intermolecular vibration of OC-³⁵Cl₂ to be 25.977637(80) cm⁻¹. These results were incorporated with other previously determined data into a spectroscopic database for generation of a five-dimensional morphed potential energy surface. This compound-model morphed potential with radial shifting (CMM-RS) was then used to make more accurate predictions of properties of the OC-³⁵Cl₂ complex including D(e) = 544(5) cm⁻¹, D₀ = 397(5) cm⁻¹, ν₃ = 56.43(4) cm⁻¹, and ν(b)¹ = 85.43(4) cm⁻¹. The CMM-RS potential determined for OC-Cl₂ was also used to compare quantitatively many of the inherent properties of this non-covalent halogen bonded complex with those of the closely related hydrogen-bonded complex OC-HCl, which has a similar dissociation energy D₀. We found that in the ground state, the CO bending amplitude is larger in OC-Cl₂ than in OC-HCl.

Infrared spectra of ethylene clusters: (C2D4)2 and (C2D4)3

Spectra of ethylene dimers and trimers are studied in the ν(11) fundamental band region of C(2)D(4) (≈2200 cm(-1)) using a tuneable quantum cascade laser to probe a pulsed supersonic slit jet expansion. The dimer spectrum is that of a prolate symmetric top perpendicular band, with a distinctive appearance because the A rotational constant is almost exactly equal to six times the B constant. The analysis supports the previously determined cross-shaped dimer structure with D(2d) symmetry. An ethylene trimer has not previously been observed with rotational resolution. The spectrum is that of an oblate symmetric top parallel band. It leads to a proposed trimer structure which is barrel shaped and has C(3h) or C(3) symmetry, with the ethylene monomer C-C axes approximately aligned along the trimer symmetry axis.

Observation of a planar isomer of the OCS–(C2H2)2 trimer

Infrared spectra of a new planar isomer of the OCS–(C2H2)2 trimer are observed in the region of the OCS ν1 fundamental band (∼2060cm−1) using a tunable diode laser to probe a pulsed supersonic slit jet expansion. Three isotopologues are analyzed in detail (16OCS–(HCCH)2, 16OCS–(DCCD)2, and 18OCS–(DCCD)2), while the spectrum of a fourth is found to be heavily perturbed (18OCS–(HCCH)2). Observed rotational constants of planar OCS–(C2H2)2 agree fairly well with previous ab initio calculations, and with previous and present semi-empirical calculations. It should now be possible to assign the microwave spectrum of planar OCS–(C2H2)2.