Ultraviolet Photodissociation of Proteinogenic Amino Acids.

The ultraviolet photochemistry of the amino acids glycine, leucine, proline, and serine in their neutral forms was investigated using parahydrogen matrix-isolation spectroscopy. Irradiation by 213 nm light destroys the chirality of all three chiral amino acids as a result of the α-carbonyl C-C bond cleavage and hydrocarboxyl (HOCO) radical production. The temporal behavior of the Fourier-transform infrared spectra revealed that HOCO radicals rapidly reach a steady state, which occurs predominantly due to photodissociation of HOCO into CO + OH or CO2 + H. In glycine and leucine, the amine radicals generated by the α-carbonyl C-C bond cleavage rapidly undergo hydrogen elimination to yield methanimine and 3-methylbutane-1-imine, respectively. Breaking of the α-carbonyl C-C bond in proline appeared to yield 1-pyrroline, although due to its weak absorption it remains unconfirmed. In serine, additional products were formaldehyde and E/Z ethanimine. The present study shows that the direct production of HOCO previously observed in α-alanine generalizes to other amino acids of varying structure. It also revealed a tendency for amino acid photolysis to form imines rather than amine radicals. HOCO should be useful in the search for amino acids in interstellar space, particularly in combination with simple imine molecules.

Hydrocarboxyl Radical as a Product of α-Alanine Ultraviolet Photolysis.

UV photodissociation of α-alanine was studied by parahydrogen matrix isolation infrared spectroscopy. The temporal behavior of Fourier transform infrared spectra revealed that UV irradiation at 213 nm yielded the HOCO radical as a direct photoproduct from the S2 excited state. The concentration of HOCO quickly approached a steady state due to secondary photodissociation of HOCO to produce CO2 + H or CO + OH. On the other hand, no photoproducts were detected by S1 excitation at 266 nm. Irradiation of fully deuterated α-alanine at 213 nm yielded ∼2 times more cis-DOCO radicals than the lower energy isomer trans-DOCO, indicating that the conformation of the hydroxyl group is fairly well-preserved upon photodissociation of α-alanine. The present study suggests that HOCO may be a good tracer species in the search for amino acids in interstellar space.

Electronic spectroscopy of Mg-phthalocyanine embedded in cold hydrogen clusters produced by a pulsed nozzle.

Cold clusters of molecular hydrogen were created using a pulsed nozzle. The thermodynamical states of the clusters were characterized by measuring the cluster beam velocity and the laser-induced fluorescence (LIF) spectra of embedded molecules. Two distinct velocity components were identified in the beam that originates from different clustering mechanisms. The fast velocity component corresponds to the expansion of H2 from the gas phase, while the slow velocity component corresponds to the expansion from the liquid phase. The velocity distribution of these two components showed no significant difference between the expansions of para and normal hydrogen. In this study, LIF spectroscopy of single Mg-phthalocyanine molecules embedded in the H2 clusters consisting of 105 H2 molecules was used to investigate the properties of the fast component. The observed peak frequencies of the LIF signals, compared to those observed in helium droplets, were used to infer the possible presence of the liquid phase in the fast component of the H2 clusters below 5 K. The shift, linewidth, and splitting in the spectra, which strongly depend on the ortho/para ratio, are attributed to the local configurations of hydrogen in the vicinity of the probe molecules.

VUV photochemistry and nuclear spin conversion of water and water-orthohydrogen complexes in parahydrogen crystals at 4 K.

Samples of H2O, HDO, and D2O were isolated in solid parahydrogen (pH2) matrices and irradiated by vacuum ultraviolet (VUV) radiation at 147 nm. Fourier-Transform Infrared (FTIR) spectra showed a clear depletion of D2O and an enrichment of both HDO and H2O by 147 nm irradiation. These irradiation-dependent changes are attributed to the production of OH and/or OD radicals through photodissociations of H2O, HDO, and D2O. The radicals subsequently react with the hydrogen matrix, leading to the observed enrichment of H2O. No trace of isolated OH or OD was detected in the FTIR spectra, indicating that the OH/OD radicals react with the surrounding matrix hydrogen molecules via quantum tunneling within our experimental timescale. The observed temporal changes in concentrations, especially the increase of HDO concentration during VUV irradiation, can be interpreted by a model with a rapid conversion from orthohydrogen (oH2) to pH2 in water-oH2 complexes upon VUV photodissociation, indicating either the acceleration of the nuclear spin conversion (NSC) of H2 due to the magnetic moment of the intermediate OH/OD radical, or the preferential reaction of the OH/OD radical with a nearby oH2 molecule over other pH2 molecules. We have also identified and quantified an anomalously slow NSC of H2O and D2O complexed with oH2 in solid pH2.

Matrix isolation spectroscopy and spectral simulations of isotopically substituted C60 molecules.

Isotopically enriched (3.5% 13C) and depleted (0.5% 13C) fullerene C60 molecules are isolated in parahydrogen (pH2) solids at cryogenic temperatures and studied by high resolution (0.01-0.1 cm-1) infrared (IR) absorption measurements. Spectra of natural isotopic abundance (1.1% 13C) C60 molecules isolated in solid pH2, orthodeuterium (oD2), and Ne matrix hosts serve to identify the relatively minor spectral perturbations due to the trapping environments. Spectral features observed for the four IR-active T1u modes of threefold degeneracy in Ih symmetry, namely, T1u(1) at 529.77 cm-1, T1u(2) at 578.24 cm-1, T1u(3) at 1184.7 cm-1, and T1u(4) at 1432 cm-1, are assigned to the superpositions of matrix perturbed vibrational-mode spectra of a number of 13Cn 12C60-n isotopologues. New molecular orbital calculations show the symmetry lowering effects of 13C substitution, namely, split vibrational frequencies and modified IR intensities. IR spectral patterns calculated for the 328 distinct isotopomers of 13Cn 12C60-n up to n = 3 are used to satisfactorily simulate most of the observed absorption features. For the T1u(4) mode at 1432 cm-1, the observed splitting is insensitive to the 13C abundance, indicating spectral perturbations due to Fermi resonance. Weak absorption features at 1545 cm-1 are assigned to a combination of lower frequency modes. We discuss relative and absolute band strengths for the astrophysical application of estimating C60 abundances in planetary nebulae.

Stability and IR Spectroscopy of Zwitterionic Form of ß-Alanine in Water Clusters.

Amino acids are the building blocks of proteins, and their detection in outer space thus has implications on the origin of life. They form a zwitterionic structure in aqueous environments while adopting a neutral configuration in the gas phase. We perform an experimental and computational study on the number of water molecules needed for zwitterion formation of ß-alanine. Our density functional theory investigation reveals that a minimum of five water molecules are required to form and stabilize the zwitterion. A characteristic connecting water molecule located between the COO- and NH3+ groups is found to enhance the stability. This water molecule is also involved in a characteristic infrared active vibration at ≈1560 cm-1, which is slightly shifted with the number of surrounding water molecules and is located in a spectral region outside of water vibrations. A corresponding infrared signal is found in high-resolution experimental spectra of ß-alanine and water in a solid para-hydrogen matrix.

Penning collisions between supersonically expanded metastable He atoms and laser-cooled Li atoms.

We describe an experimental setup comprised of a discharge source for supersonic beams of metastable He atoms and a magneto-optical trap (MOT) for ultracold Li atoms that makes it possible to study Penning ionization and associative ionization processes at high ion count rates. The cationic reaction products are analyzed using a novel ion detection scheme which allows for mass selection, a high ion extraction efficiency, and a good collision-energy resolution. The influence of elastic He-Li collisions on the steady-state Li atom number in the MOT is described, and the collision data are used to estimate the excitation efficiency of the discharge source. We also show that Penning collisions can be directly used to probe the temperature of the Li cloud without the need for an additional time-resolved absorption or fluorescence detection system.

Production of rotationally cold methyl radicals in pulsed supersonic beams.

We present a comparison of two technically distinct methods for the generation of rotationally cold, pulsed supersonic beams of methyl radicals (CH3): a plate discharge source operating in the glow regime and a dielectric barrier discharge source. The results imply that the efficiency of both sources is comparable and that molecular beams with similar translational and rotational temperatures are formed. Methane (CH4) proved to be the most suitable radical precursor species.

Diffusion of Water Molecules in Quantum Crystals.

With the use of solid parahydrogen in matrix isolation spectroscopy becoming more commonplace over the past few decades, it is increasingly important to understand the behavior of molecules isolated in this solid. The mobility of molecules in solid parahydrogen can play an important role in the dynamics of the system. Water molecules embedded in solid parahydrogen as deposited were found to be mobile at 4.0 K on the time scale of a few days. The diffusion at this temperature must be due to quantum tunneling in solid parahydrogen. The diffusion dynamics were analyzed based on the theory of nucleation. The concentration dependence on the diffusion rate indicates that there might be correlated motion of water molecules, a signature of quantum diffusion. We find that both water monomers and water dimers migrate in solid parahydrogen and provide insight into the behavior of molecules embedded in this quantum crystal.

Magnetic Trapping of Cold Methyl Radicals.

We have demonstrated that a supersonic beam of methyl radicals (CH_{3}) in the ground rotational state of both para and ortho species has been slowed down to standstill with a magnetic molecular decelerator, and successfully captured spatially in an anti-Helmholtz magnetic trap for >1 s. The trapped CH_{3} radicals have a mean translational temperature of about 200 mK with an estimated density of >5.0×10^{7} cm^{-3}. The methyl radical is an ideal system for the study of cold molecules not only because of its high reactivities at low temperatures, but also because further cooling below 1 mK is plausible via sympathetic cooling with ultracold atoms. The demonstrated trapping capability of methyl radicals opens up various possibilities for realizing ultracold ensembles of molecules towards Bose-Einstein condensation of polyatomic molecules and investigations of reactions governed by quantum statistics.

Longitudinal Spin Relaxation of Optically Pumped Rubidium Atoms in Solid Parahydrogen.

We have grown crystals of solid parahydrogen using a single closed-cycle cryostat. We have doped the crystals with rubidium atoms at densities on the order of 10^{17} cm^{-3} and used optical pumping to polarize the spin state of the implanted atoms. The optical spectrum of the rubidium atoms shows larger broadening than previous work in which the rubidium was implanted in solid argon or neon. However, the optical pumping behavior is significantly improved, with both a larger optical pumping signal and a longer longitudinal relaxation time. The spin relaxation time shows a strong dependence on orthohydrogen impurity levels in the crystal, as well as the applied magnetic field. Current performance is comparable to state-of-the-art solid state systems at comparable spin densities, with potential for improvement at higher parahydrogen purities.

Ro-vibrational Spectra of (para-H2 )N -CH4 in He Droplets.

In this work, we report on the infrared spectroscopic study of clusters of CH4 molecules with up to N=80 para-hydrogen molecules assembled inside He droplets. Upon increase of the number of the added para-hydrogen molecules up to about N=12, both the rotational constant, B, and the origin frequency of the υ3 band of CH4 decrease gradually. In the range of 6 ≤N≤12, the spectra indicate some abrupt changes of B and υ3 with both values being approximately constant at N≥12. The origin of this effect is discussed. Comparison of the spectra of methane molecules in para-hydrogen clusters to that in solid para-hydrogen is also presented.

UV photochemistry of benzene and cyclohexadienyl radical in solid parahydrogen.

Matrix-isolation spectroscopy in solid parahydrogen is a powerful technique for the study of photochemical reactions in detail. It is especially suited for the detection of chemical intermediates and unstable molecular products of photochemical reactions that are otherwise difficult to observe using other conventional spectroscopic techniques. Here, we have revisited UV photochemistry of benzene induced by 193 and 253.7 nm excitations by matrix-isolation infrared spectroscopy using solid parahydrogen as a host matrix. In addition to the formation of benzvalene, fulvene, and Dewar benzene as photoproducts of the UV photolysis of benzene, we have confirmed the production of cyclohexadienyl radical in solid parahydrogen as an intermediate species for a ring-opening reaction to 1,3,5-hexatriene. Moreover, we have identified 1,3-hexadien-5-yne and o-benzyne as minor products of the photochemical reaction. The reaction mechanisms of these species based on the analysis of FTIR spectra and quantum chemical calculations are discussed.

Infrared spectroscopy and phase behavior of n-butane aerosols and thin films at cryogenic temperatures.

Spectroscopic studies of two phase transitions of solid n-butane aerosol droplets performed under conditions representative of those in the lower atmosphere of Titan are presented. Pure n-butane aerosols and mixed ensembles of n-butane/acetylene, n-butane/carbon dioxide and n-butane/water aerosols were generated in a bath gas cooling cell at 78 K and their phase transition dynamics monitored using infrared extinction spectroscopy. For pure n-butane aerosols the volume and surface nucleation constants were found to range from JV = 10(12) -10(14) cm(-3) s(-1) and JS = 10(13) -10(15) cm(-2) s(-1), respectively, for the first observed transition, and JV = 10(9) -10(11) cm(-3) s(-1) and JS = 10(11) -10(13) cm(-2) s(-1) for the second observed transition. The phases of the n-butane aerosols were determined by comparing their spectroscopic signatures with spectra collected from thin films of liquid and solid n-butane. The first observed transition was from an amorphous-annealed phase into the metastable crystalline phase II of n-butane. The second transition was from the metastable crystalline phase II into the crystalline phase III. The effect of the presence of a second aerosol substance (acetylene, carbon dioxide or water) was examined; while this accelerated the first phase transition, it did not directly influence the rate of the second phase transition. The kinetic studies might be important for the understanding of cloud formation on Titan, while the spectral data provided, which include the first reported spectrum of liquid n-butane close to the melting point, are expected to be of use for remote sensing applications.

Infrared spectroscopy of chloromethyl radical in solid parahydrogen and its nuclear spin conversion.

We present high-resolution infrared absorption spectra of chloromethyl radical produced by in situ UV photolysis of chloroiodomethane isolated in solid parahydrogen. The radicals were stable over a few days in solid parahydrogen kept at 3.6 K. Analysis of the rotation-vibration spectra revealed that the radical exhibited quantized one-dimensional rotational motion about the C-Cl bond, while the ortho and para nuclear spin species were still clearly distinguishable in the spectra. Temporal change of the spectra indicated that the nuclear spin conversion between the ortho and para nuclear spin species of the radical in solid parahydrogen occurred in a time scale of a few hours at 3.6 K. It was also found that the nuclear spin conversion became significantly slower in a higher concentration of chloroiodomethane.

Manipulation of translational motion of methyl radicals by pulsed magnetic fields.

Control of the translational motion of methyl radicals is demonstrated by using pulsed magnetic fields created by a series of solenoid coils. By synchronously switching the magnetic fields corresponding to the motion of the radicals, we have observed the deceleration and focusing of a supersonic radical beam. The present result shows that Zeeman deceleration of polyatomic free radicals in the doublet state can be realized. This opens possibilities for trapping cold polyatomic free radicals in a spatially confined area leading to further studies of cold collisions and reactions.

Cluster size resolving analysis of CH3F-(ortho-H2)n in solid para-hydrogen using FTIR absorption spectroscopy at 3 µm region.

Infrared absorption spectra of methyl fluoride with ortho-hydrogen (ortho-H(2)) clusters in a solid para-hydrogen (para-H(2)) crystal at 3.6 K were studied in the C-H stretching fundamental region (~3000 cm(-1)) using an FTIR spectrometer. As shown previously, the ν(3) C-F stretching fundamental band of CH(3)F-(ortho-H(2))(n) (n = 0, 1, 2, ...) clusters at 1040 cm(-1) shows a series of n discrete absorption lines, which correspond to different-sized clusters. We observed three unresolved broad peaks in the C-H stretching region and applied this cluster model to them assuming the same intensity distribution function as the ν(3) band. A fitting analysis successfully gave us the linewidth and lineshift of the components in each vibrational band. It was found that the separately determined linewidth, matrix shift of the band origin, and cluster shift are dependent on the vibrational mode. From the transition intensities of the monomer component derived from the fitting analysis, we discuss the mixing ratio of the vibrational modes due to Fermi resonance.

Excitation and emission spectra of rubidium in rare-gas thin-films.

To understand the optical properties of atoms in solid state matrices, the absorption, excitation, and emission spectra of rubidium doped thin-films of argon, krypton, and xenon were investigated in detail. A two-dimensional spectral analysis extends earlier reports on the excitation and emission properties of rubidium in rare-gas hosts. We found that the doped crystals of krypton and xenon exhibit a simple absorption-emission relation, whereas rubidium in argon showed more complicated spectral structures. Our sample preparation employed in the present work yielded different results for the Ar crystal, but our peak positions were consistent with the prediction based on the linear extrapolation of Xe and Kr data. We also observed a bleaching behavior in rubidium excitation spectra, which suggests a population transfer from one to another spectral feature due to hole-burning. The observed optical response implies that rubidium in rare-gas thin-films is detectable with extremely high sensitivity, possibly down to a single atom level, in low concentration samples.

Laser induced fluorescence spectroscopy of tetracene with large Ar, Ne, and H2 clusters in superfluid He nanodroplets.

Clusters of tetracene molecules with different numbers of attached (Ar)(N), (Ne)(N) and (H(2))(N) particles (N = 1-2000) are assembled inside superfluid He nanodroplets and studied via laser-induced fluorescence. The frequency shift of the fluorescence spectrum of the tetracene molecules is studied as a function of cluster size and pickup order of tetracene and cluster species. For (Ar)(N) and (Ne)(N) clusters, our results indicate that the tetracene molecules reside inside the clusters when tetracene is captured by the He nanodroplet before the cluster species; conversely, the tetracene molecules stay on the surface of the clusters when tetracene is captured after the cluster species. In the case of (H(2))(N) clusters, however, tetracene molecules reside inside the (H(2))(N) clusters irrespective of the pickup order. We conclude that (Ar)(N) and (Ne)(N) clusters are rigid at T = 0.38 K, while (H(2))(N) clusters of up to N = 2000 remain fluxional at the same temperature. The results may also indicate the occurrence of heterogeneous nucleation of the (H(2))(N) clusters, which is induced by the interaction with tetracene chromophore molecules.

Refractive index measurements of solid parahydrogen.

Solid para-H2 is a promising gain medium for stimulated Raman scattering, due to its high number density and narrow Raman linewidth. In preparation for the design of a cw solid hydrogen Raman laser, we have made the first measurements, to our knowledge, of the index of refraction of a solid para-H2 crystal, in the wavelength range of 430-1100 nm. For a crystal stabilized at 4.4 K, this refractive index is measured to be n(p-H2)=1.130±0.001 at 514 nm. A slight, but significant, dependence on the final crystal-growth temperature is observed, with higher n(p-H2) at higher crystal-growth temperatures. Once a crystal is grown, it can be heated up to 10 K with no change in n(p-H2). The refractive index varies only slightly over the observed wavelength range, and no significant birefringence was observed.