High-resolution infrared spectra of mass selected protonated methane, CH₅⁺, have been recorded in the C-H stretching region in a 22-pole ion trap experiment at low temperatures. The frequencies of the infrared OPO system (pump and signal) have been calibrated using a NIR frequency comb. As a result the ro-vibrational IR transition frequencies of CH₅⁺ could be determined to an accuracy in the MHz regime.O. Asvany, J. Krieg, and S. Schlemmer, Frequency comb assisted mid-infrared spectroscopy of cold molecular ions, Review of Scientific Instruments, 83 (2012), 076102.n this contribution we discuss different techniques of laser induced reactions which enabled recording spectra at different temperatures.O. Asvany, S. Brünken, L. Kluge, and S. Schlemmer, COLTRAP: a 22-pole ion trapping machine for spectroscopy at 4 K, Applied Physics B: Lasers and Optics, 114 (2014), 203-211he spectra simplify dramatically at a nominal trap temperature of 4 K. Nevertheless an assignment of these spectra is very difficult. We apply the idea of the Rydberg-Ritz combination principle to the complex spectra of protonated methane in order to get first hints at the energy level structure of this enigmatic molecule.

---

Footnotes:

O. Asvany, J. Krieg, and S. Schlemmer, Frequency comb assisted mid-infrared spectroscopy of cold molecular ions, Review of Scientific Instruments, 83 (2012), 076102.I O. Asvany, S. Brünken, L. Kluge, and S. Schlemmer, COLTRAP: a 22-pole ion trapping machine for spectroscopy at 4 K, Applied Physics B: Lasers and Optics, 114 (2014), 203-211T

Producing ultracold molecules is the first step in precision molecular spectroscopy. Here we present some of the challenges and advantages of SiO+ as well as some of our progress toward meeting those challenges. To demonstrate ground state SiO+, we first load about 100 SiO+ via 2+1 REMPI into an ion trap. Translational motion of SiO+ is then sympathetically cooled by co-trapped Ba+, which is laser cooled. To prepare the population into the ground state, we optically pump the P-branch (rotational cooling transitions) in the B:Σ(v’=0) ← X:Σ(v=0) band with broadband radiation. Because the band is highly diagonal, population can be effectively driven into the rotational ground state before falling into other manifolds. The broadband source, a fs laser, is spectrally filtered using an ultrashort pulse shaping technique to drive only the P-branch. Attention must be paid when aligning the optics to obtain sufficient masking resolution. We have achieved 3 cm⁻¹ resolution, which is sufficient to modify a broadband source for rotationally cooling SiO+.

The bichromatic optical force (BCF), which can greatly exceed radiative forces, seems ideal for laser slowing and cooling of molecules because it minimizes the effects of radiative decay. However, it relies on sustained coherences between optically coupled states, and molecules, with their many sublevels and decay pathways, present new challenges in maintaining these coherences compared with simple atoms. We have conducted extensive numerical simulations of BCFs in model molecular systems based on the B ↔ X transition in CaF, and have begun experimental tests in a molecular beam.
In our modeling, the effects of fine and hyperfine structure are examined using a simplified level scheme that is still sufficiently complete to include the major pathways leading to loss or decoherence. To circumvent optical pumping into coherent dark states we explore two possible schemes: (1) a skewed dc magnetic field, and (2) rapid optical polarization switching. The effects of repumping to compensate for out-of-system radiative decay are also examined. Our results verify that the BCF is a promising method for creating large forces in molecular beams while minimizing out-of-system radiative losses, and provide detailed guidance for experimental designs. Compared to a two-level atom, the peak force is reduced by about an order of magnitude, but there is little reduction in the velocity range over which the force is effective. Our experiments on deflection and slowing using the CaF B ↔ X, (0-0) transition, still at an early stage, include studies of both the P₁₁(1.5)/^PQ₁₂(0.5) branch, a quasi-cycling configuration with extensive hfs, and the R₁₁(0.5)/^RQ₂₁(0.5) branch, which has a much simpler hfs but requires rotational repumping.

Solid para-H₂ is a popular accommodating host for impurity spectroscopy due to its unique softness and the spherical symmetry of para-H₂ in its J=0 rotational level. T.\ Momose, H. Honshina, M. Fushitani and H. Katsuki, Vib. Spectrosc.\ 34, 95(2004).^, M. E. Fajardo, J. Phys.\Chem. A 117, 13504 (2013).To simulate the properties of impurity−dopedsolid para−H_2, a reliable model for the `soft′pure solidpara−H_2 at different pressures is highly desirable. While acouple of experimental I. F. Silvera, Rev. Mod. Phys.\52, 393(1980).nd theoretical F. Operetto and F.\Pederiva, Rhys. Rev. B 73, 184124(2006).tudies aimed atelucidating the equation of state (EOS) of solid para−H_2 havebeen reported, the calculated EOS was shown to be heavily dependenton the potential energy surface (PES) between two para−H_2 thatwas used in the simulations. T. Omiyinka and M. Boninsegni,Rhys. Rev. B 88, 024112(2013).he current study alsodemonstrates that different choices of the parameters governing the Quantum MonteCarlo simulation could produce different EOS curves.To obtain a reliable model for pure solid para−H_2, we used a new1−D para−H_2 PES reported by Faruk et al. N.\Faruk, M. Schmidt, H. Li, R. J. Le Roy, and P.−N. Roy,J.\Chem. Phys. bf 141, 014310(2014).hat was obtained by averagingover Hinde′s highly accurate 6−D H_2-H_2 PES. R. J.\Hinde, J. Chem. Phys. 128, 154308(2008).The EOS of puresolid para−H_2 was calculated using the PIMC algorithm withperiodic boundary conditions (PBC). To precisely determine the equilibriumdensity of solid para−H_2, both the value of the PIMC time step\) and the number of particles in the PBC cell were extrapolated to convergence. The resulting EOS agreed well with experimental observations, and the hcp structured solid para-H₂ was found to be more stable than the fcc one at 4.2K, in agreement with experiment. The vibrational frequency shift of para-H₂ as a function of the density of the pure solid was also calculated, and the value of the shift at the equilibrium density is found to agree well with experiment.

---

Footnotes:

 T.\ Momose, H. Honshina, M. Fushitani and H. Katsuki, Vib. Spectrosc.\ 34, 95(2004).

The only report in the literature on the infrared spectroscopy of the parent oxynitrene NOH was performed using Ar matrix isolation spectroscopy at 10 K.G. Maier, H. P. Reisenauer, M. De Marco, Angew. Chem. Int. Ed. 38, 108-110 (1999).n this previous study, they performed detailed isotopic studies to make definitive vibrational assignments. NOH is predicted by high-level calculations to be in a triplet ground electronic state,U. Bozkaya, J. M. Turney, Y. Yamaguchi, and H. F. Schaefer III, J. Chem. Phys. 136, 164303 (2012).ut the Ar matrix isolation spectra cannot be used to verify this triplet assignment. In our 2013 preliminary report,David T. Anderson and Mahmut Ruzi, 68th Ohio State University International Symposium on Molecular Spectroscopy, talk TE01 (2013).e showed that 193 nm in situ photolysis of NO trapped in solid parahydrogen can also be used to prepare the NOH molecule. Over the ensuing two years we have been studying the infrared spectroscopy of this species in more detail. The spectra reveal that NOH can undergo hindered rotation in solid parahydrogen such that we can observe both a-type and b-type rovibrational transitions for the O-H stretch vibrational mode, but only a-type for the mode assigned to the bend. In addition, both observed a-type infrared absorption features (bend and OH stretch) display fine structure; an intense central peak with weaker peaks spaced symmetrically to both lower and higher wavenumbers. The spacing between the peaks is nearly identical for both vibrational modes. We now believe this fine structure is due to spin-rotation interactions and we will present a detailed analysis of this fine structure. Currently, we are performing additional experiments aimed at making ¹⁵NOH to test these preliminary assignments. The most recent data and up-to-date analysis will be presented in this talk.

---

Footnotes:

G. Maier, H. P. Reisenauer, M. De Marco, Angew. Chem. Int. Ed. 38, 108-110 (1999).I U. Bozkaya, J. M. Turney, Y. Yamaguchi, and H. F. Schaefer III, J. Chem. Phys. 136, 164303 (2012).b David T. Anderson and Mahmut Ruzi, 68th Ohio State University International Symposium on Molecular Spectroscopy, talk TE01 (2013).w

The absorption spectrum of the ν₃ (C-F stretching) mode of CH₃F in solid para-H₂ by FTIR showed a series of equal interval peaksK. Yoshioka and D. T. Anderson, J. Chem. Phys. 119 (2003) 4731-4742 Their interpretation was that the n-th peak of this series was due to CH₃F-(ortho-H₂)_n clusters which were formed CH₃F and n’s ortho-H₂ in first nearest neighbor sites of the para-H₂ crystal with hcp structure. In order to understand this system in more detail, we have studied these peaks, especially n = 0 – 3 corresponding to 1037 - 1041 cm⁻¹, by using high-resolution and high-sensitive infrared quantum cascade (QC) laser spectroscopy. Before now, we found many peaks around each n-th peak of the cluster, which we didn’t know their originsA. R. W. McKellar, A. Mizoguchi, and H. Kanamori, J. Chem. Phys. 135 (2011) 124511 We observed photochromic phenomenon of these peaks by taking an advantage of the high brightness of the laserA. R. W. McKellar, A. Mizoguchi, and H. Kanamori, Phys. Chem. Chem. Phys. 13 (2011) 11587-11589 In this study, we focus on satellite series consisting of six peaks which locate at the lower energy side of each main peak. All the peaks showed a common red shouldered line profile, which corresponds to partly resolved transitions of ortho- and para- CH₃F. The spectral pattern and time behavior of the peaks may suggest that these satellite series originate from a family of CH₃F clusters involving ortho-H₂ in second nearest neighbor sites. A model function assuming this idea is used to resolve the observed spectrum into each Lorentzian component, and then some common features of the satellite peaks are extracted and the physical meanings of them will be discussed.

---

Footnotes:

K. Yoshioka and D. T. Anderson, J. Chem. Phys. 119 (2003) 4731-4742. A. R. W. McKellar, A. Mizoguchi, and H. Kanamori, J. Chem. Phys. 135 (2011) 124511. A. R. W. McKellar, A. Mizoguchi, and H. Kanamori, Phys. Chem. Chem. Phys. 13 (2011) 11587-11589.

Singlet dihydroxycarbene (HOCOH) is produced via pyrolytic decomposition of oxalic acid, captured by helium nanodroplets, and probed with infrared laser Stark spectroscopy. Rovibrational bands in the OH stretch region are assigned to either trans,trans- or trans,cis- rotamers on the basis of symmetry type, nuclear spin statistical weights, and comparisons to electronic structure theory calculations. Stark spectroscopy provides the inertial components of the permanent electric dipole moments for these rotamers. The dipole components for trans,trans- and trans,cis- rotamers are (μ_a, μ_b) = (0.00, 0.68(6)) and (1.63(3), 1.50(5)), respectively. The infrared spectra lack evidence for the higher energy cis,cis- rotamer, which is consistent with a previously proposed pyrolytic decomposition mechanism of oxalic acid and computations of HOCOH torsional interconversion and tautomerization barriers.

Chlorine atoms, generated through the thermal decomposition of Cl₂, are solvated in superfluid helium nanodroplets and clustered with HCl molecules. The H–Cl stretching modes of these clusters are probed via infrared laser spectroscopy. A broad band centered at ≈ 2880.9 cm⁻¹ is assigned to the binary Cl···HCl complex. The band center is red shifted by only 7.4 cm⁻¹ from the “free” HCl stretch (ν₁) of (HCl)₂ and, as such, is consistent with an assignment to a similarly “free” HCl stretch. Also, the breadth of the band ( ≈ 2 cm⁻¹ FWHM) is consistent with assignment to a mostly b-type component of the H–Cl stretch; the band is lifetime broadened to a similar extent as the predominantly b-type ν₁ stretch of (HCl)₂, due to fast rotational relaxation facilitated by the helium droplet environment. Despite the lack of rotational structure, which would verify our assignment, the spectrum is consistent with stabilization of a weakly-bound complex having an L-shaped geometry. Computations reveal that the projection of the transition dipole moment onto the a-axis results in a dramatic decrease ( ≈ 700 times) in the intensity of the a-type band relative to the b-type band intensity; indeed, the signal-to-noise ratio in our experiment precluded observation of an a-type band for this complex. No bands were observed that could derive from a strongly H-bonded Cl···HCl complex. Additionally, we located two bands at 2764.0 and 2798.5 cm⁻¹ that are consistent with the pick-up of two HCl molecules and are therefore assigned to vibrations of the Cl···(HCl)₂ complex.

Helium nanodroplet spectroscopy is a well-established experimental technique to study weakly bound complexes and reactive species. The superfluid helium interacts weakly with the embedded species, leading to only small matrix-induced shifts in vibrational spectra. This technique has been applied for the spectroscopic study of the resonance-stabilized allyl radical and its reactions and complexes.C. M. Leavitt, C. P. Moradi, B. W. Acrey, G. E. Douberly; J. Chem. Phys. 2013, 139, 234301.^,D. Leicht, D. Habig, G. Schwaab, M. Havenith; J. Phys. Chem. A 2015, 119, 1007.he tropyl radical is another example of a D. Leicht, D. Habig, G. Schwaab, M. Havenith; J. Phys. Chem. A 2015, 119, 1007.TE. P. F. Lee, T. G. Wright; J. Phys. Chem. A 1998, 102, 4007.T

In bulk aqueous solutions the interactions between solute and solvent are still not fully understood. We apply spectroscopy in Helium nanodroplets to investigate solvation processes step by step (bottom up approach). Recently, the Bochum helium nanodroplet spectrometer has been equipped with a quantum cascade laser spanning the frequency ranges from 1000-1400, 1600-1700, and 2500-2600 cm⁻¹. First results with the extended setup will be presented.

The van der Waals complex of H₂O with CO₂ has attracted considerable theoretical interest since it is a typical example of a weak binding complex (less than 3 kcal/mol), but a very few IR data are available in gas. For these reasons, we have studied in solid neon hydrogen bonded complexes involving carbon dioxide and water molecules. Evidence for the existence of at least three (CO₂)_m(H₂O)_n, or m:n, complexes has been obtained from the appearance of many new absorptions near the well-know monomers fundamental transitions. Concentration effects and detailed vibrational analysis allowed identification of fifteen, eleven and four transitions for the 1:1, 1:2, and 2:1 complexes, respectively. Careful examination of the far infrared allows the assignment of several 1:1 and 1:2 intermolecular modes, confirmed by the observation of combinations of intra+intermolecular transitions. All of these results significantly increase the number of one and, especially, two quanta vibrational transitions observed for these complexes, and anharmonic coupling constants have been derived. This study shows the high sensibility of the solid neon isolation for the spectroscopy of the hydrogen-bonded complexes since two quanta transitions can’t be easily observed in gas phase.

The [2C, 2N, S] and the [2C, 2N, Se] systems were investigated by quantum chemical computations and matrix isolation IR spectroscopy. For both systems nine isomers were computationally investigated, for which harmonic and anharmonic vibrational wavenumbers and infrared (IR) intensities were calculated using the CCSD(T)/aug-cc-pVTZ level of theory. The results show that each of the isomers have two or more detectable bands in the mid IR region, which have one or two orders of magnitude larger intensity compared to the IR intensity of the most intense bands of the most stable NCSCN and NCSeCN isomers’. It follows that if the most stable isomers can be detected, then the other previously unobserved isomers generated from NCSCN or NCSeCN should also be detectable with IR spectroscopy. UV spectra were also computed for each isomer at the TD-DFT B3LYP/aug-cc-pVTZ level of theory. These computations showed that the most stable isomers (NCSCN and NCSeCN) can absorb the UV radiation around 250 nm, and the irradiation may promote photoisomerization. This means that if the initial isomers are irradiated by narrow-band UV radiation, new isomers may be generated, which likely decompose by irradiating broad-band UV radiation. The two most stable isomers, sulphur dicyanide (NCSCN) and selenium dicyanide (NCSeCN), were prepared following literature methods. The matrix isolation IR spectra of these molecules in Ar and Kr were measured for the first time. As a result of a selective 254 nm-irradiation of the deposited matrices some new bands appeared in the IR spectra, while the intensity of the bands of NCSCN or NCSeCN were decreased at the same time. Irradiation of the matrices with broad-band UV light decreased the intensity of the bands corresponding to the deposited isomers and some of the bands appeared on the 254 nm-irradiation. On the basis of the analysis of the formation rates of the different bands upon 254 nm photolysis and by comparison with the results of the quantum chemical calculations these bands could be assigned to new isomers. In the case of sulphur analogue NCSNC and NCNCS were unambiguously identified, and for selenium analogue the formation of NCSeNC and NCNCSe isomers were observed.

Acetylenic carbenes and conjugated carbon chain molecules of the HC_nH family are relevant to the study of combustion and chemistry in the interstellar medium (ISM). Propynylidene (HC₃H) has been thoroughly studied and its structure and photochemistry determined.Seburg, R. A.; Patterson, E. V.; McMahon, R. J., Structure of Triplet Propynylidene (HCCCH) as Probed by IR, UV/vis, and EPR Spectroscopy of Isotopomers. Journal of the American Chemical Society 2009, 131 (26), 9442-9455.ere, we produce triplet diradical 1,3-dimethylpropynylidene (MeC₃Me) photochemically from a precursor diazo compound in a cryogenic matrix (N₂ or Ar) at 10 K, and spectroscopic analysis is carried out. The infrared, electronic absorption, and electron paramagnetic resonance spectra were examined in light of the parent (HC₃H) system to ascertain the effect of alkyl substituents on delocalized carbon chains of this type. Computational analysis, EPR, and infrared analysis indicate a triplet ground state with a quasilinear structure. Infrared experiments reveal photochemical reaction to penten-3-yne upon UV irradiation. Further experimental and computational results pertaining to the structure and photochemistry will be presented.

---

Footnotes:

Seburg, R. A.; Patterson, E. V.; McMahon, R. J., Structure of Triplet Propynylidene (HCCCH) as Probed by IR, UV/vis, and EPR Spectroscopy of Isotopomers. Journal of the American Chemical Society 2009, 131 (26), 9442-9455.H

Co-depositions of methanol (CH₃OH) and benzene (C₆H₆) in an argon matrix at 20 K result in the formation of a 1:1 methanol-benzene complex (CH₃OH-C₆H₆) as evidenced by the observation of distinct infrared bands attributable to the complex near the O-H, C-H, and C-O stretching fundamental vibrations of CH₃OH and the hydrogen out-of-plane bending fundamental vibration of C₆H₆. Co-deposition experiments were also performed using isotopically labeled methanol (CD₃OD) and benzene (C₆D₆) and the corresponding deuterated complexes were also observed. Based on ab initio and density functional theory calculations, the structure of the complex is thought to be an H-π complex in which the CH₃OH is above the C₆H₆ ring with the OH hydrogen atom interacting with the π cloud of the ring. Close inspection of the O-H and O-D stretching peaks of the complexes reveals small, distinct satellite peaks that are approximately 3 – 4 cm⁻¹lower than the primary peak. A series of experiments have been performed to ascertain the nature of the satellite peaks. These consist of co-depositions in which the concentrations of both monomers were varied over a large range (1:200 to 1:1600 S/M ratios), annealing experiments (20 K to 35 K), and lower temperature cycling experiments (20 K to 8 K). Based on the results of these experiments, it is concluded that the satellite peaks are due to rotational structure and not due to matrix site effects, higher aggregation or distinct complex geometries. Given the rigidity of a low temperature argon matrix, it is proposed that the rotational motion responsible for the satellite peaks is internal rotation within the methanol subunit of the complex rather than overall molecular rotation of the complex.