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Imidazole (IM) is an important moiety, having a proton donor and acceptor, in nucleic acid bases, i.e., adenine and guanine, and plays an important role in the intermolecular interactions of biological processes so that many theoretical and experimental studies of IM and its clusters have been investigated. Previously, we have presented and characterized the high-resolution infrared laser spectra of IM, imidazole dimer (IMD) and two isomers of IMand IMDwater complexes (IMW and IMDW) formed in helium nanodroplets. Recently, we have extended the study of imidazole clusters to imidazole trimer (IMT) reporting the vibrational spectra of IMT in the free NH spectral region and found out that the IM molecules tend to form linear structures due to their dipole-dipole interactions in helium nanodroplets. The unambiguous free NH band assignment was made by the experimental techniques, such as pick-up oven temperature dependence and vibrational transition moment angle (VTMA), and density functional theory (DFT) calculations. In the present note, higher order complexes, IMT with one water molecule (IMTWs), formed in He nanodroplets are presented. To the best of our knowledge, there is no report on the spectroscopic studies of IMTWs systems due to experimental difficulties in obtaining well resolved spectra. In this study we have used advantage of using helium nanodroplets since the very weak interactions between the target molecules and the helium give rise to almost unperturbed environments from that of the gas phase. Furthermore, the temperature of the helium nanodroplets is extremely low, 0.37 K, thus the spectral broadening is significantly reduced making the He droplets as an ideal matrix for infrared spectroscopy. Using the helium nanodroplet techniques for obtaining well resolved spectra, we were able to measure the angle between the transition and permanent dipole moments of the target molecules, called the vibrational transition moment angles (VTMAs), which turned out to be a very sensitive structural and unambiguous vibrational band assignment tool. Furthermore, VTMAs can be also easily calculated and compared with the experimental values obtained in this study. The use of VTMAs for the specific tautomer and vibrational band assignments of small biomolecules is well described in our previous reports. In this note we present the first spectroscopic investigation on the IMTW systems applying the VTMA techniques in He nanodroplets. Figure 1 shows the two lowest energy structures for IMTWs (> NH...OH2 and > N: ...H-O-H), where IMT is acting as a proton acceptor and donor from the nitrogen atom of the IM ring in the two lowest energy isomers, IMTW1 and 2, respectively. The structures were optimized at the B3LYP/6-311++G(d,p) level of theory and the calculated vectors representing the directions of the permanent electric dipole moments (solid arrows) and the vibrational transition moments (empty arrows) are superimposed onto the clusters. The magnitudes of the various moments for IMTWs are given in Table 1. It is clear from the figure that the patterns of VTMAs for the two isomers are quite different, making them a useful tool for distinguishing these

We report the first vibrational band assignment of imidazole trimer (IMT) solvated in helium nanodroplets. Several congested vibrational bands of imidazole (IM) clusters were obtained in the frequency region of and vibrationally resolved due to the extremely low temperature (0.37 K) and very weak solutesolvent interaction environments of helium droplets. The unambiguous free NH band assignment was achieved with an aid of pick-up oven temperature dependence and vibrational transition moment angle (VTMA) experiments as well as density functional theory (DFT) calculations. The band at is attributed to the free NH stretching mode of linear IMT clusters, easily formed by the dipole-dipole interactions of IM in ultracold helium nanodroplets.

Summary In this study we have presented the first electric field depen-dence of IR spectra of cytosine isolated in superfluid helium nanodroplets. By varying a DC electric field to the polar mole-cules, the different permanent dipole moments of system in-duced different orientation behavior with respect to the polari-zation and electric field intensity. Using the recently developed technique by this group and more precise measurement of the band ratio, we have reanalyzed the measurement of VTMA for the NH 2 (SS) mode of C1, 85 o ± 5 o rather than 74 o ± 5 o , pre-viously measured in this group. The new value is in excellent agreement with the ab initio value, 88 o . Other bands are in good agreement between experiments, both the VTMA and dipole moment curve measurement, and theory. We have attempted to characterize the nonplanarity of cyto-sine monomers with an aid of the experimental VTMAs of each vibrational band as in the case of adenine. Contrast to the nonplanarity study of adenine, the experimental VTMA change as a function of the dihedral angle, N

Mid-infrared spectra are employed to identify and characterize the structure and specific vibrational modes of xanthine monomer in helium nanodroplets. The predicted NH vibrational frequencies of xanthine show strong dependence on the level of theory and the basis set used in this study. However, the measurement of the angle between the permanent and corresponding vibrational transition dipole moment, called the vibrational transition moment angle (VTMA), gives unambiguous and straightforward assignments to the specific vibrational bands of xanthine. In this work, the global minimum tautomer of xanthine is observed and characterized using ab initio calculations and VTMAs for the various vibrational modes of the xanthine monomer.

Infrared (IR)–IR double resonance spectroscopy is used to probe the isomerization dynamics of helium solvated HF–N2O following the vibrational excitation of one of two stable isomers on the intermolecular potential energy landscape. An upstream pump laser excites the H–F stretching fundamental of either the linear ONN–HF or the bent NNO–HF isomer, while a downstream probe laser is scanned through the bands of both isomers to test for photo-induced isomerization. Vibrational excitation of the H–F stretch mode leads to the isomerization of a fraction of the upstream population, regardless of the identity of the pumped isomer. The measured isomer interconversion is consistent with vibrational predissociation followed by geminate recombination. A fraction of the upstream population is unaccounted for by the downstream probe. We propose that a fraction of the missing population is due to the mismatch in the timescales of complex recombination and pump, probe events.

Mid-infrared spectra are reported for adenine monomer in helium nanodroplets. We show that there is only one tautomer of adenine, the global minimum structure, observed in helium nanodroplets and characterized by using ab initio calculations and the measurement of vibrational transition moment angles (VTMAs) for the various vibrational modes of the adenine monomer. On the basis of the VTMA analysis on the amino group of the global minimum tautomer, which gives insights into its nonplanarity, a detailed VTMA study of three lowest-energy amino tautomers of adenine is discussed in this study.

Prereactive metal atom-HCN entrance channel complexes [M-HCN (M=Al, Ga, In)] have been stabilized in helium nanodroplets. Rotationally resolved infrared spectra are reported for the CH stretching vibration of the linear nitrogen-bound HCN-Ga and HCN-In complexes that show significant perturbation due to spin-orbit coupling of the 2Pi1/2 ground state with the 2Sigma1/2 state which are degenerate at long range. Six unresolved bands are also observed and assigned to the linear hydrogen-bound isomers of Al-HCN, Ga-HCN, and In-HCN corresponding to the fundamental CH stretching vibration and a combination band involving the CH stretch plus intermolecular stretch for each isomer. A nitrogen-bound HCN-Al complex is not observed, which is attributed to reaction, even at 0.37 K. This conclusion is supported by the observation of a weakly bound complex containing two HCN's and one Al atom which, from the analysis of its rotationally resolved zero-field and Stark spectra is assigned to a weakly bound complex of a HCNAl reaction product and a second HCN molecule. Theoretical calculations are presented to elucidate the reaction mechanisms and energetics of these metal atom reactions with HCN.

High-resolution infrared spectroscopy has been used to determine the structures, C-H stretching frequencies, and dipole moments of the HCN-Agn (n = 1-3) complexes formed in superfluid helium droplets. The HCN-Ag4 cluster was tentatively assigned based upon pick-up cell pressure dependencies and harmonic vibrational shift calculations. Ab initio and density functional theory calculations were used in conjunction with the high-resolution spectra to analyze the bonding nature of each cluster. All monoligated species reported here are bound through the nitrogen end of the HCN molecule. The HCN-Agn complexes are structurally similar to the previously reported HCN-Cun clusters, with the exception of the HCN-Ag binary complex. Although the interaction between the HCN and the Agn clusters follows the same trends as the HCN-Cun clusters, the more diffuse nature of the electrons surrounding the silver atoms results in a much weaker interaction.

Infrared-infrared double resonance spectroscopy is used to probe the vibrational dynamics of molecular complexes solvated in helium nanodroplets. We report results for the acetylene-HCN and cyanoacetylene-HCN binary complexes, each having two stable isomers. We find that vibrational excitation of an acetylene-HCN complex results in a population transfer to the other isomer. Photoinduced isomerization is found to be dependent on both the initially excited vibrational mode and the identity of the acetylene-HCN isomer. However, population transfer is not observed for the cyanoacetylene-HCN complexes. The results are rationalized in terms of the ab initio intermolecular potential energy surfaces for the two systems with particular emphasis on the long-range barriers to rearrangement.

Infrared laser spectroscopy has been used to characterize imidazole (IM), imidazole dimer (IMD), and imidazole-water (IMW) binary systems formed in helium nanodroplets. The experimental results are compared with ab initio calculations reported here. Vibrational transition moment angles provide conclusive assignments for the various complexes studied here, including IM, one isomer of IMD, and two isomers of the IMW binary complexes.

Infrared laser spectroscopy is used to study the four lowest energy tautomers of guanine, isolated in helium nanodroplets. The large number of vibrational bands observed in the infrared spectrum are assigned by comparing the corresponding experimental vibrational transition moment angles with those obtained from ab initio theory. The result is the conclusive assignment of the spectrum to the N9H-Keto, N7H-Keto, N9Ha-Enol(trans), and N9Hb-Enol(cis) tautomers. The dipole moments of these tautomers are also experimentally determined and compared with ab initio theory.

High-resolution infrared spectra have been obtained in the 2.7 μm region of water monomer, solvated in superfluid helium nanodroplets. A total of five ro-vibrational transitions have been identified for the ν3 bands (OH stretch) of H2O and HDO. The associated Lorentzian line widths fall in the range 0.05–2.0 cm−1 (FWHM). The electric field induced Q(0) transition of HDO has also been observed. In the latter case the broadening appears to be inhomogeneous, with a line width of 0.007 cm−1. The line widths for the various transitions correlate with the available rotational energy in the excited state, relative to the energies of the elementary excitations of the liquid helium.

High-resolution infrared laser spectroscopy has been used to obtain rotationally resolved spectra of HCN-Zn(n) (n = 1-4) complexes formed in helium nanodroplets. In the present study the droplets passed through a metal oven, where the zinc vapor pressure was adjusted until one or more atoms were captured by the droplets. A second pickup cell was then used to dope the droplets with a single HCN molecule. Rotationally resolved infrared spectra are obtained for all of these complexes, providing valuable information concerning their structures. Stark spectra are reported and used to determine the corresponding permanent electric dipole moments. Ab initio calculations are also reported for these complexes for comparison with the experimental results.

Helium nanodroplets have emerged as a new and exciting medium for studying the structure and dynamics of both this quantum solvent and impurities that can be doped into (onto) and grown inside (on the surface) of the droplets. Spectroscopic studies of these molecular impurities can provide detailed information on helium as a solvent and its interaction with the solute. This is particularly important given that helium is completely transparent to photons below 20 eV, making the direct spectroscopic study of liquid helium problematic. Since liquid helium is an extremely weak solvent, the corresponding perturbations to the spectrum of the solute molecules are often minor; really only evident because of the high resolution that is often achieved in such studies. As a result, helium nanodroplet spectra often resemble the corresponding gas-phase results. Indeed, for the case of rotational spectroscopy, the gas-phase Hamiltonian is often sufficient to describe the system, with the effects of the solvent being to...

Electron impact ionization of a helium atom in a helium nanodroplet is followed by rapid charge migration, which can ultimately result in the localization of the charge on an atomic or molecular solute. This process is studied here for the cases of hydrogen cyanide, acetylene, and cyanoacetylene in helium, using a new experimental method we call optically selected mass spectrometry (OSMS). The method combines infrared laser spectroscopy with mass spectrometry to separate the contributions to the overall droplet beam mass spectrum from the various species present under a given set of conditions. This is done by vibrationally exciting a specific species that exists in a subset of the droplets (for example, the droplets containing a single HCN molecule). The resulting helium evaporation leads to a concomitant reduction in the ionization cross sections for these droplets. This method is used to study the charge migration in helium and reveals that the probability of charge transfer to a solvated molecule does not approach unity for small droplets and depends on the identity of the solvated molecule. The experimental results are explained quantitatively by considering the effect of the electrostatic potential (between the charge and the embedded molecule) on the trajectory of the migrating charge.

Two IR lasers are used in a pump–probe configuration to observe photo-induced isomerization between the linear and bent isomers of HCN–HF, formed in helium nanodroplets. Vibrational excitation of the C–H and H–F stretching modes of these complexes provides sufficient energy to surmount the barriers between them. The extent of population transfer is found to be different for pumping the two isomers. In the case of linear HCN–HF, the results suggest that the complex undergoes vibrational predissociation, followed by geminate recombination. Excitation of the higher energy bent HF–HCN isomer results in complete population transfer to the linear complex. This isomer specific population transfer provides important clues regarding the associated vibrational dynamics.

High-resolution infrared laser spectroscopy is used to study acetylene-magnesium complexes formed in helium nanodroplets. The rotational fine structure observed in the spectrum of the binary complex is well described by a gas-phase Hamiltonian and is used to determine that the structure is T-shaped, with the magnesium binding weakly to the triple bond of the acetylene. The binary complex has a dipole moment of 0.39 ( 5, as measured by Stark spectroscopy. Difference electron density maps reported herein reveal that this dipole moment arises from a strong polarization of the magnesium charge cloud by the quadrupole moment of the acetylene. Preliminary spectra are also reported for acetylene -Mgn (n ) 2, 3). Although these are not fully rotationally resolved, comparisons with ab initio calculations reported here provide some insights into the nature of these complexes.

High-resolution infrared laser spectroscopy has been used to determine the structures of HCN-Mgn complexes formed in helium nanodroplets. The magnesium atoms are first added to the droplets to ensure that the magnesium complexes are preformed before the HCN molecule is added. The vibrational frequencies, structures, and dipole moments of these complexes are found to vary dramatically with cluster size, illustrating the nonadditive nature of the HCN-magnesium interactions. All of the complexes discussed here have the nitrogen end of the HCN pointing towards the magnesium clusters. For Mg3, the HCN binds to the "threefold" site, yielding a symmetric top spectrum. Although the HCN-Mg4 complex also has C3v symmetry, the HCN sits "on-top" of a single magnesium atom. These structures are confirmed by both ab initio calculations and measurements of the dipole moments. Significant charge transfer is observed in the case of HCN-Mg4, indicative of charge donation from the lone pair on the nitrogen of HCN into the lowest unoccupied molecular orbital of the Mg4.

Infrared-infrared double resonance spectroscopy is used as a probe of the vibrational dynamics of cyanoacetylene in helium droplets. The nu1 C-H stretching vibration of cyanoacetylene is excited by an infrared laser and subsequent vibrational relaxation results in the evaporation of approximately 660 helium atoms from the droplet. A second probe laser is then used to excite the same C-H stretching vibration downstream of the pump, corresponding to a time delay of approximately 175 micros. The hole burned by the pump laser is narrower than the single resonance spectrum, owing to the fact that the latter is inhomogeneously broadened by the droplet size distribution. The line width of the hole is characteristic of another broadening source that depends strongly on droplet size.

The infrared spectrum of formic acid dimers in helium nanodroplets has been observed corresponding to excitation of the "free" OH and CH stretches. The experimental results are consistent with a polar acyclic structure for the dimer. The formation of this structure in helium, as opposed to the much more stable cyclic isomer with two O-H...O hydrogen bonds, is attributed to the unique growth conditions that exist in helium droplets, at a temperature of 0.37 K. Theoretical calculations are also reported to aid in the interpretation of the experimental results. At long range the intermolecular interaction between the two monomers is dominated by the dipole-dipole interaction, which favors the formation of a polar dimer. By following the minimum-energy path, the calculations predict the formation of an acyclic dimer having one O-H...O and one C-H...O contact. This structure corresponds to a local minimum on the potential energy surface and differs significantly from the structure observed in the gas phase.

Infrared laser spectroscopy and ab initio calculations are reported for cyanoacetylene bound to magnesium clusters of varying size, formed and observed in the superfluid helium nanodroplets. Ab initio calculations of the binary HCCCN-Mg potential energy surface reveal three minima. The corresponding isomers are observed experimentally, owing to the unique "growth" conditions provided by liquid helium. The cyanoacetylene is also observed to bind at both the nitrogen and hydrogen ends to larger magnesium clusters in agreement with the minima found in the corresponding ab initio calculations. The infrared spectra show rotational structure that can be fit by the corresponding gas-phase Hamiltonian, using modified constants that account for the effects of the helium solvent. Stark spectroscopy is used to measure the dipole moments of these complexes, providing further insights into the nature of the associated interactions.

Rotationally resolved vibrational spectra are reported for a total of 15 hydrogen-HF clusters formed in helium nanodroplets, each with a different size (up to 6 hydrogens), or isotopic (H2/D2) or o...

High resolution infrared laser spectroscopy has been used to obtain rotationally resolved vibrational spectra of diactyelene and cyclopropane in helium nanodroplets. In part, this work was motivated by the need for a large database of rotational constants for molecules solvated in liquid helium. Data of this type provides benchmarks with which to test the theoretical methods that are currently being developed for calculating the effect of the helium on the rotational dynamics of these solvated molecules. In general, the correlated motion of the helium and the molecule results in an effective moment of inertia that is considerably larger than that of the gas phase molecule.

Rapid laser-induced thermal ablation of aerosol particles can result in the ejection of desolvated ions into the gas phase. In an effort to understand this phenomenon, we have carried out studies of ethylene glycol droplets containing 10-6 and 10-1 M RbCl. A high-power infrared CO2 laser pulse vaporized the aerosol particles, and the resulting ions were detected by time-of-flight mass spectrometry and by total current measurements. The absolute number of ions in the particle was varied by controlling the size and the concentration of the ions in the particles. The results show that, at low concentrations, the ion signal is directly proportional to the ion concentration, whereas the signal saturates above 10-4 M RbCl. By comparing the TOF signal to the total current measured on the ion repeller plate, we were able to determine that ion−ion recombination is the major cause of this signal saturation. The ion collection efficiency of the TOF mass spectrometer at low ion concentration, where ion−ion recombinat...

The low temperatures and high resolution associated with helium droplet spectroscopy are used to obtain vibrational spectra for clusters containing a single HF molecule, surrounded by multiple H2, D2, and HD molecules. For clusters with fewer than nine HD molecules, the asymmetric solvent environment experienced by the HF prevents it from internally rotating. The results suggest that, for n = 9 and larger, the solvent cage becomes highly isotropic, allowing the HF molecule to undergo nearly free rotation. For H2 and D2 the mixture of ortho and para species in the clusters makes the solvent cage anisotropic even when the first solvent shell is complete and free rotation of the HF molecule is not observed.

The reactive uptake of gases by a particle is explored by numerically solving the associated partial differential equations. These equations explicitly couple the diffusion and reaction of multiple species in the particle. This approach makes it possible to calculate the concentration of each reactant within the particle as a function of both time and position and to examine the impact of concentration gradients on the rate of uptake. The effect of liquid diffusion on the rate of uptake is explored for a reference reaction (O3 + oleic acid particles) as a function of the particle size and the liquid-state concentration of the species taken up from the gas phase. Diffusion within the particle is found to significantly limit uptake for large particles and the large gas-phase concentrations often used in laboratory experiments, although it is typically less important under atmospheric conditions. This numerical approach to solving the diffusion−reaction equations is also used to modify the common electric ci...

Rotationally resolved infrared spectra are reported for the T-shaped acetylene–HF complex in helium nanodroplets, corresponding to excitation of both the H–F and asymmetric C–H stretches. In comparison to the isolated complex, rotation about the A-axis is relatively unhindered by the helium, while overall rotation is slower by a factor of approximately two. An ab initio potential surface is also reported for this system. Although the bent complex, with acetylene acting as a hydrogen donor, is determined to be a local minimum, only the T-shaped isomer is experimentally observed, indicating that the barrier is too small to prevent the metastable bent structure from rearranging to the global minimum.

High-resolution infrared spectra are reported for ortho- and para-H2, ortho- and para-D2, and HD binary complexes with HF, solvated in helium nanodroplets. These rotationally resolved spectra provide detailed information on the rotational and vibrational dynamics of these systems. The para-H2–HF complex is observed, indicative of the unusual growth conditions present in the droplets. In the case of the HD–HF complex, extensive line broadening is observed, which we tentatively explain in terms of the near resonance between the HF and HD vibrational frequencies, which presumably facilitates vibrational relaxation.

High-resolution infrared spectra are reported for two isomers of the magnesium–HCN binary complex, corresponding to the magnesium bonding at the nitrogen and hydrogen ends of the molecule. Stark spectra are also reported for these complexes, from which the corresponding dipole moments are determined. Ab initio calculations confirm that the potential energy surface has two minima, consistent with the experimentally determined structures. The wave functions of the two dimensional intermolecular coupled cluster singles doubles (triples) potential energy surface, calculated with the collocation method, are also reported for the magnesium–HCN system and are used to calculate the vibrationally averaged dipole moment for both isomers, which are then compared to experiment.

We report the first experimental observation of (HF)n (n = 4,5,6) clusters, grown in helium nanodroplets. The structures of these complexes are determined from infrared laser spectroscopy, recorded...

Although laser vaporization of aerosol particles plays an important role in aerosol mass spectrometry, relatively little is known about disposal of excess energy in the degrees of freedom of the gas-phase species. A two-laser scheme, in which an infrared laser vaporizes aerosol particles prior to ionization of the vapor plume by a vacuum-ultraviolet laser, permits the determination of the internal energy of the neutral molecules created in laser vaporization. In this work, the fragmentation patterns of vacuum UV (VUV) photoionization mass spectra of ethylene glycol, in conjunction with photoelectron-photoion coincidence (PEPICO) measurements, determines the internal energy of gas-phase molecules created in the CO2 laser vaporization of neat ethylene glycol particles and ethanol particles mixed with trace ethylene glycol. The internal energy ranges from 1300 to 10250 cm-1 for CO2 laser powers between 25 and 112 mJ/pulse. For neat ethylene glycol, the rate with which the internal and kinetic energy grows wi...

The technique of single-particle mass spectrometry has been coupled to a reaction flow tube to measure the uptake coefficient, γ, of ozone (O3) by oleic acid (9-octadecenoic acid) aerosol particles. The reaction was followed by monitoring the decrease of oleic acid in the size-selected particles as a function of O3 exposure. The reactive uptake coefficient is found to depend on the size of the particle, with γmeas ranging from (7.3 ± 1.5) × 10-3 to (0.99 ± 0.09) × 10-3 for particles ranging in radius from 680 nm to 2.45 μm. It is suggested that the decrease in γmeas with increasing particle size results from the reaction being limited by the diffusion of oleic acid within the particle, and based on our measurements we estimate the value of γ to be (5.8−9.8) × 10-3 for particles that are not limited by oleic acid diffusion. A reaction model that includes simultaneous diffusion and reaction of both O3 and oleic acid is developed and used to fit the observed rates of reaction. Solutions obtained from this mo...

The growth of water clusters in liquid helium droplets results in the formation of cyclic structures up to and including the hexamer. In view of the sequential nature of the molecular pick-up process, the formation of water rings involves the insertion of water monomers into preformed cyclic water clusters. The implication of this observation is that the barriers to the ring insertion process are low enough to be overcome during the experiment. This paper presents a combined experimental and theoretical effort to explore the insertion process in detail. Our results provide important new insights into the dynamics of hydrogen-bonded networks. We map out the cluster potential energy surfaces and visualize them using disconnectivity graphs. Nonequilibrium walks on these surfaces show that ring water clusters can be formed during sequential addition of water molecules by surmounting small barriers that are thermally accessible even at the low temperature of the experiment. We find that the effects of zero-point energy are significant in making these processes feasible.

We present here the first measurements of the infrared complex refractive indices for supercooled ternary solutions of H2SO4, HNO3, and H2O under stratospheric conditions. The data sets were retrieved directly from the infrared extinction spectra of laboratory-generated aerosols. Comparisons of these results with a previously reported empirical mixing rule model1 reveal significant differences, which we trace to errors in the binary acid/water thin film refractive indices used in the model. In addition, the model does not properly account for the concentrations of ionic and neutral species in the ternary solutions. The experimentally determined optical constants reported here will be useful for determining polar stratospheric cloud aerosol properties from infrared spectroscopic remote sensing measurements.

Single aerosol particles of ethylene glycol and oleic acid are vaporized on a heater at temperatures between 500 and 700 K, and the resulting vapor plume is ionized by a 10.5-eV vacuum ultraviolet (VUV) laser. The mass spectra are compared to those obtained by CO2 laser vaporization followed by VUV laser ionization. The relative intensities of the parent and fragment ion peaks are remarkably similar for the two modes of vaporization. A Maxwell-Boltzmann distribution of speeds accurately describes the dependence of the signal as a function of the VUV laser pulse timing. The signal levels obtained with this design are sufficient to obtain good-quality mass spectra.

Rotationally resolved spectra of h2- and d1-formic acid embedded in liquid helium droplets have been recorded in the spectral region of the O–H (ν1) and C–H (ν2) stretch vibrations. Accidental resonant mixing between the ν1-band of h2-formic acid and the combination bands (ν2+ν7) and (ν2+ν9) has been observed. The fundamental ν1 band can interact via Fermi resonance with the (ν2+ν7), and the (ν2+ν7) via Coriolis coupling with the (ν2+ν9) band. Examination of the resonance induced line-broadening effects suggests that the helium environment modifies vibrational relaxation dynamics. The 2ν3 C=O stretch overtone is also observed and was assigned by a density functional theory (DFT) ab initio calculation. The spectroscopic constants are determined by fitting all spectra with a standard gas phase Hamiltonian.

High resolution infrared laser spectroscopy has been used to study the rotational and vibrational dynamics of methane in helium nanodroplets. Because of the nuclear spin symmetry, four transitions (P(1),Q(1),R(0) and R(1)) are observed. While the first three have widths of approximately 0.2 cm −1 , which we attribute to moderately fast vibrational relaxation, the significantly larger width of the R(1) transition suggests even faster rotational relaxation for the latter. At higher methane pressures in the pick-up cell, three bands are observed that are attributed to clusters of methane. The spectra suggest that nearly free rotation of the monomer sub-units is still occurring in these larger clusters.

Rotationally resolved infrared spectra are reported for the linear and T-shaped isomers of acetylene–hydrogen cyanide formed in helium nanodroplets. The results for the latter show that the overall rotation ((B+C)/2) is slowed by the helium, while rotation about the A-axis is only weakly affected. A comparison of the relative abundances of the two isomers formed in helium and in a gas phase free jet expansion provides insights into the nature of the growth in helium.

Laser-initiated decomposition of single aerosol particles in the source region of a time-of-flight spectrometer, followed by vacuum ultraviolet laser ionization and mass analysis of the reaction products, reveals the identity of species formed in the early stages of condensed-phase decomposition in nitromethane and nitromethane−diethylamine mixtures. In pure nitromethane, the initial stages of decomposition are characterized by unimolecular processes. At near-threshold energies, the cleavage of the C−N bond to form CH3 and NO2 dominates the dynamics, and at higher energies we find evidence of rearrangement to methylnitrite, followed by the formation of CH3O and NO. In nitromethane seeded with 0.1% diethylamine, an additional channel is observed which produces ionic intermediates. Detection of protonated diethylamine and the nitromethane aci-anion confirm that an acid−base reaction is involved in this case. We also detect the radical anion, NO2-, whose production is correlated with an increase in methyl ra...

High-resolution optothermal laser spectroscopy is reported for the C-H stretching vibrations of the binary complexes formed between monodeuterated acetylene and carbon monoxide and nitrogen. In contrast with a previous study of the corresponding C(2)H(2) complexes, where the C-H fundamental spectra were highly perturbed [R. D. Beck, A. G. Maki, S.-H. Tseng, and R. O. Watts, J. Mol. Spectrosc. 158, 306-317 (1993)], the present spectra are well described by a simple linear-rotor Hamiltonian. This is presumed to result from the fact that the C-H vibration in the monodeuterated complexes is decoupled from the intermolecular degrees of freedom. Copyright 2000 Academic Press.

Microwave-infrared double-resonance spectroscopy has been used to probe the solvation environment and its influence on the rotational relaxation of a cyanoacetylene molecule embedded in a superfluid 4He nanodroplet. The results support a model in which (within any given rotational state) the guest molecules are distributed over a set of spectroscopically inequivalent states which are most likely “particle-in-a-box” states originating from the confinement of the guest molecule within the droplet. Revisitation of previously collected microwave–microwave double-resonance data suggests that transitions between these states occur at a rate which is comparable to the rotational relaxation rate, but not fast enough as to produce motionally narrowed, homogeneous absorption lines. The relative intensities of the rotational lines in the microwave-infrared double-resonance spectra are observed to depend strongly on the average droplet size. In the large droplet limit we can explain the observed pattern by invoking a...

The rotationally resolved infrared spectrum of the HF stretch fundamental in combination with one quantum of internal rotation excitation has been investigated for the HF–BF3 complex using a color-center-laser optothermal molecular-beam spectrometer. The infrared band origin is red-shifted by 3.9 cm−1 from the HF monomer origin. The magnitude of the Coriolis interaction in the m=±1 internal rotor state observed in combination with the HF stretch gives a barrier to internal rotation of 54 cm−1 in the HF stretch excited state.

Frequency-dependent optical constants have been determined from the Fourier transform infrared spectra of laboratory-generated liquid sulfuric acid/water aerosols over a range of temperatures and compositions that are relevant to the upper troposphere and lower stratosphere of Earth. The compositions of the particles were determined in situ using a tunable diode laser to monitor equilibrium water vapor pressures. The infrared complex refractive indices of sulfuric acid are shown to be strongly dependent on temperature and composition, because of changes in the equilibrium between sulfate and bisulfate ions. Results from this study also have implications in understanding the temperature dependence of intermolecular interactions within ionic solutions. The database presented here is the most extensive yet available for the liquid solutions of sulfuric acid.

The optothermal method has been used to record a rotationally resolved near-infrared spectrum of the carbon monoxide–water binary complex. The vibrational band studied here corresponds to the asymmetric O–H water stretch. All of the transitions in the spectrum are doubled, due to water tunneling within the complex. The present results are used, together with those obtained from previous studies of this complex, to draw several interesting conclusions about the nature of the tunneling dynamics in this system.

Reported here are three aspects of molecular stereochemistry associated with polar molecules in an exotic solvent, namely, superfluid liquid helium. Infrared spectroscopy is used to study the nature of the solvent–solute and solute–solute interactions in this unique environment. First, we use an external dc electric field to orient the helium solvated polar molecules in the laboratory frame of reference and we characterize this orientation using pendular state spectroscopy. Second, we present results that demonstrate non-equilibrium self-assembly of polar molecules into linear chain structures, due to the mutual orientation imposed by the associated dipole–dipole interactions. Finally, the spectroscopic results provide information on the anisotropic interactions experienced by the solvated molecules, which result in significant molecular orientation in the droplet frame of reference. Studies of this type provide important new insights into the nature of the interactions between the molecules and the superfluid solvent.

Complex refractive indices have been retrieved directly from the FT-IR extinction spectra of laboratory-generated, binary sulfuric acid/water aerosols over a range of temperatures and compositions relevant to the atmosphere of Earth. The refractive indices are strongly dependent upon both temperature and composition, due to changes in the equilibrium between sulfate and bisulfate ions and the resultant change in the corresponding solvation interactions. The present results have important implications for determining the composition of sulfuric acid aerosols by remote sensing applications.

The vibrational predissociation dynamics of HCN–HF have been studied at the state-to-state level by recording photofragment angular distributions resulting from excitation of both the C–H and H–F stretching vibrations. The results obtained from a combination of a parent molecule orientation technique and photofragment probe laser experiments show that most of the excess energy is found in rotational energy of the HF fragment. Nevertheless, in contrast with other systems studied to date, HCN–HF is very unusual in that a significant fraction of the available energy appears in the form of recoil kinetic energy (∼40%). The dissociation energy of the complex is also determined to be 1970±10 cm −1 .

Frequency-dependent complex refractive indices have been determined from the aerosol infrared extinction spectra of laboratory-generated nitric acid dihydrate (NAD) particles. The spectra reported here were recorded at several temperatures (160, 180, and 190 K) over the mid-infrared region of the spectrum (700−4700 cm-1). The optical constants retrieved from these spectra are overall in reasonable agreement with results from thin-film experiments conducted at 184 K, although there are significant differences in the intensities of several absorption bands located below 1500 cm-1. We attribute these to the different methods used to make the samples in the two types of experiments and consider the substrate-free aerosol results to be unperturbed NAD. The temperature dependence of the NAD aerosol optical constants was found to be quite weak between 160 and 190 K, in contrast to the strong temperature dependence observed previously for the optical constants of water ice.

The rotationally resolved infrared spectrum is reported for the C–H stretching vibration of Ar–HCCD. A bending combination band is also observed from which we obtain an accurate value for the low-frequency intermolecular bending frequency. The vibrational predissociation lifetime of Ar–HCCD is found to be considerably longer than that of Ar–C 2 H 2 . This difference is attributed to the strong Fermi resonance in Ar–C 2 H 2 which is absence in Ar–HCCD. We propose that the perturbing level in the Ar–C 2 H 2 complex provides a `doorway' that accelerates the dissociation in this system. In Ar–HCCD, where this `doorway state' is not Fermi mixed, the dissociation is slower.