UV spectra of benzene isotopomers and dimers in helium nanodroplets

We report spectra of various benzene isotopomers and their dimers in helium nanodroplets in the region of the first Herzberg-Teller allowed vibronic transition 6(0)(1) (1)B(2u)<--(1)A(1g) (the A(0) (0) transition) at approximately 260 nm. Excitation spectra have been recorded using both beam depletion detection and laser-induced fluorescence. Unlike for many larger aromatic molecules, the monomer spectra consist of a single "zero-phonon" line, blueshifted by approximately 30 cm(-1) from the gas phase position. Rotational band simulations show that the moments of inertia of C(6)H(6) in the nanodroplets are at least six-times larger than in the gas phase. The dimer spectra present the same vibronic fine structure (though modestly compressed) as previously observed in the gas phase. The fluorescence lifetime and quantum yield of the dimer are found to be equal to those of the monomer, implying substantial inhibition of excimer formation in the dimer in helium.     

Hydrogen bonding in substituted formic acid dimers

The hydrogen-bonded dimers of formic acid derivatives XCOOH (X = H, F, Cl, and CH3) have been investigated using density functional theory (B3LYP) and second-order M?ller-Plesset perturbation (MP2) methods, with the geometry optimization carried out using 6-311++G(2d,2p) basis set. The dimerization energies calculated using aug-cc-pVXZ (with X = D and T) basis have been extrapolated to infinite basis set limit using the standard methodology. The results indicate that the fluorine-substituted formic acid dimer is the most stable one in comparison to the others. Topological analysis carried out using Bader's atoms in molecules (AIM) theory shows good correlation of the values of electron density and its Laplacian at the bond critical points (BCP) with the hydrogen bond length in the dimers. Natural bond orbital (NBO) analysis carried out to study the charge transfer from the proton acceptor to the antibonding orbital of the X-H bond in the complexes reveals that most of the dimers are associated with conventional H-bonding except a few, where improper blue-shifting hydrogen bonds are found to be present.     

High-resolution IR spectroscopy of dimers of HDO with H2O in helium nanodroplets

We report on rotationally resolved IR spectra of dimers of HDO as a deuterium (d) donor with H(2)O, HDO, and D(2)O embedded in superfluid Helium nanodroplets in the 2650-2660 and 2725-2740 cm(-1) regions of the O-D donor stretch and symmetric acceptor stretch vibrations, respectively. By comparing spectra at different levels of deuteration we were able to unambiguously assign the donor stretch signals of H(2)O···DOH, HDO···DOH, and D(2)O···DOH. For H(2)O···DOH, three ΔK(a) = 0 sub-bands were found that were assigned to transitions from the lower and upper acceptor switching states of K(a) = 0 and the lower acceptor switching state of K(a) = 1. In addition, b- and c-type transitions in the acceptor stretch region of HDO···DOH were observed that allowed us to determine the acceptor switching splitting of Δv? = 5.68 cm(-1) in the HDO···DOH vibrational ground state. We suggest that the dominating broadening mechanism is intervibrational relaxation due to coupling of the rovibrational levels of the chromophore via internal droplet excitations.     

Partition potential for hydrogen bonding in formic acid dimers

The ground-state energy and density of 4 low-energy conformations of the formic acid dimer were calculated via partition density functional theory (PDFT). The differences between isolated and PDFT monomer densities display similar deformation patterns for primary and secondary hydrogen bonds (HBs) among all 4 dimers. In contrast, the partition potential shows no transferable features in the bonding regions. These observations highlight the global character of the partition potential and the cooperative effect that occurs when a dimer is bound via more than 1 HB. We also provide numerical confirmation of the intuitive (but unproven) observation that fragment deformation energies are larger for systems with larger binding energies.     

The isomers of HF-HCN formed in helium nanodroplets: Infrared spectroscopy and ab initio calculations

Binary complexes containing hydrogen cyanide and hydrogen fluoride are formed in helium nanodroplets, and studied using high-resolution infrared laser spectroscopy. Rotationally resolved spectra are reported for the H-F and C-H stretches of the linear HCN-HF complex, a system that has been thoroughly studied in the gas phase. We report the high-resolution spectra of the higher energy, bent HF-HCN isomer, which is also formed in helium. Stark spectra are reported for both isomers, providing dipole moments of these complexes. The experimental results are compared with ab initio calculations, also reported here. Spectra are reported for several ternary complexes, including (HCN)2-HF, HCN-(HF)2, HF-(HCN)2, and HF-HCN-HF.     

Ionization of methane clusters in helium nanodroplets

The electron ionization of helium droplets doped with methane clusters is investigated for the first time using high-resolution mass spectrometry. The dominant ion products ejected into the gas phase are the unprotonated (CH(4))(n)(+) cluster ions along with the protonated ions, CH(5)(+)(CH(4))(n-1). The mass spectra show clear evidence for magic numbers, which are broadly consistent with icosahedral shell closings. However, unusual features were observed, including different magic numbers for CH(5)(+)(CH(4))(n-1) (n=55, 148) when compared to (CH(4))(n)(+) (n=54, 147). Possible interpretations for some of these differences are proposed. Products of the type [C(2)H(x)(CH(4))(n)](+), which result from ion-molecule chemistry, are also observed and these too show clear magic number features. Finally, we report the first observation of (CH(4))(n)(2+) dications from methane clusters. The threshold for dication survival occurs at n≥70 and is in good agreement with a liquid droplet model for fission of multiply charged ions. Furthermore, we present evidence showing that these dications are formed by an unusual two-step mechanism which is initiated by charge transfer to generate a monocation and is then followed by Penning ionization to generate a dication.     

Electron impact ionization of haloalkanes in helium nanodroplets

Electron impact (EI) mass spectra of a selection of C1-C3 haloalkanes in helium nanodroplets have been recorded to determine if the helium solvent can significantly reduce molecular ion fragmentation. Haloalkanes were chosen for investigation because their EI mass spectra in the gas phase show extensive ion fragmentation. There is no evidence of any major softening effect in large helium droplets ( approximately 60 000 helium atoms), but some branching ratios are altered. In particular, channels requiring C-C bond fission or concerted processes leading to the ejection of hydrogen halide molecules are suppressed by helium solvation. Rapid cooling by the helium is not sufficient to account for all the differences between the helium droplet and gas phase mass spectra. It is also suggested that the formation of a solid "snowball" of helium around the molecular ion introduces a cage effect, which enhances those fragmentation channels that require minimal disruption to the helium cage for products to escape.     

Interchange-tunneling splitting in HCl dimer in helium nanodroplets

Midinfrared spectra of HCl dimers have been obtained in helium nanodroplets. The interchange-tunneling (IT) splitting in the vibrationally excited state of the bonded H-Cl stretching band (nu(2)) in (H(35)Cl-H(37)Cl) dimers was measured to be 2.7+/-0.2 cm(-1), as compared to 3.7 cm(-1) in free dimer. From the splitting, the strength of the IT coupling in liquid helium of 0.85+/-0.15 cm(-1) was obtained, which is about a factor of 2 smaller than in the free dimer. The results are compared with the previous spectroscopic study of (HF)(2) in He droplets as well as the theoretical study of (HF)(2) and (HCl)(2) dimers in small He clusters.     

Spectroscopy and dynamics of barium-doped helium nanodroplets

Excitation spectra up to the ionization threshold are reported for barium atoms located on the surface of helium nanodroplets. For states with low principal quantum number, the resonances are substantially broadened and shifted towards higher energy with respect to the gas phase. This has been attributed to the repulsive interaction of the excited atom with the helium at the Franck-Condon region. In contrast, for states with high principal quantum number the resonances are narrower and shifted towards lower energies. Photoelectron and ZEKE spectroscopy reveal that the redshift results from a lowering of the ionization threshold due to polarization of the helium by the barium ionic core. As a result of the repulsive interaction with the helium, excited barium atoms desorb from the surface of the droplets. Only when excited to the 6s6p (1)P(1) state, which reveals an attractive interaction with the helium, the atoms remain attached to the droplets.     

Mid-infrared spectroscopy of molecular ions in helium nanodroplets

High resolution IR spectra of aniline, styrene, and 1,1-diphenylethylene cations embedded in superfluid helium nanodroplets have been recorded in the 300-1700 cm(-1) range using a free-electron laser as radiation source. Comparison of the spectra with available gas phase data reveals that the helium environment induces no significant matrix shift nor leads to an observable line broadening of the resonances. In addition, the IR spectra have provided new and improved vibrational transition frequencies for the cations investigated, as well as for neutral aniline and styrene. Indications have been found that the ions desolvate from the droplets after excitation by a non-evaporative process in which they are ejected from the helium droplets. The kinetic energy of the ejected ions is found to be ion specific and to depend only weakly on the excitation energy.     

Electronic spectroscopy of biphenylene inside helium nanodroplets

We have recorded the S1 <-- S0 electronic spectra of Biphenylene and its Ar and O2 van der Waals complexes inside helium nanodroplets using beam depletion detection. In general, the spectrum is similar to the previously reported high-resolution REMPI spectrum. The zero phonon lines, however, are split similar to the previously reported tetracene case. The calculated potential energy surface predicts that helium atoms can simultaneously occupy all equivalent global minima positions. Therefore, it appears that the splitting cannot be explained either by different isomers or by tunneling. Furthermore, surprisingly the splitting is retained for the Ar van der Waals complexes (and possibly for the O2 complex as well). This case suggests that the current models of the origin of zero phonon line splitting and the helium solvation are incomplete.     

Spectroscopy of Cs attached to helium nanodroplets

Cesium oligomers are formed on helium nanodroplets which are doped with one or a few Cs atoms. The monomer absorption of the first electronic p<--s transition upon laser excitation is probed. Spectra employing laser-induced fluorescence, beam depletion, and resonant photoionization are compared. In particular, mass-resolved photoionization allows us to specifically probe excitation induced processes such as, e.g., the formation of cesium-helium exciplexes. Absorption spectra of Cs dimers and trimers are recorded in the spectral region accessible by a Ti:sapphire laser. Assignment of dimer spectra is achieved by comparison with model calculations based on ab initio potentials. Electronic absorption lines of Cs trimers are attributed to transitions in the quartet manifold.     

Ion-molecule reactions and fragmentation patterns in helium nanodroplets

A study has been made of the ion chemistry of a series of small molecules that have been embedded in helium nanodroplets. In most instances, the molecules H2O, SO2, CO2, CH3OH, C2H5OH, C3H7OH, CH3F, and CH3Cl have been allowed to form clusters, and reactivity within these has been initiated through electron impact ionization. For two of the molecules studied, CF2Cl2 and CF3I, reactivity is believed to originate from single molecules embedded in the droplets. Electron impact on the droplets is thought to first create a helium ion, and formation of molecular ions is then assumed to proceed via a charge hopping mechanism that propagates though the droplet and terminates with charge-transfer to a molecule or cluster. The chemistry exhibited by many of the cluster ions and at least one of the single molecular ions is very different from that observed for the same species in isolation. In most cases, reactivity appears to be dominated by high-energy bond breaking processes as opposed to, in the case of the clusters, ion-molecule reactions. Overall, charge-transfer from He+ does not appear to be a "soft" ionization mechanism.     

Excited state dynamics of Ag atoms in helium nanodroplets

The excited state dynamics of silver atoms embedded in helium nanodroplets have been investigated by a variety of spectroscopic techniques. The experiments reveal that 5p 2P1/2 <-- 5s 2S1/2 excitation of embedded silver atoms results almost exclusively in the ejection of silver atoms populating the 2P1/2 state. In contrast, excitation to the 5p 2P3/2 state leads to the ejection of not only silver atoms in the 2P1/2, 2P3/2, and 2D5/2 excited states but also of AgHe and AgHe2. These AgHe exciplexes are mainly formed in the A2Pi1/2 electronic state. In addition, it is found that a considerable fraction of the 2P3/2 excited silver atoms become solvated within the helium droplets, most probably as AgHe2. The observations can be accounted for by a model in which the metastable 2D5/2 state of silver acts as a doorway state in the relaxation of 2P3/2 excited silver atoms.     

Photoionization and photofragmentation of SF6 in helium nanodroplets

The photoionization of He droplets doped with SF6 was investigated using tunable vacuum ultraviolet (VUV) synchrotron radiation from the Advanced Light Source (ALS). The resulting ionization and photofragmentation dynamics were characterized using time-of-flight mass spectrometry combined with photofragment and photoelectron imaging. Results are compared to those of gas-phase SF6 molecules. We find dissociative photoionization to SF5+ to be the dominant channel, in agreement with previous results. Key new findings are that (a) the photoelectron spectrum of the SF6 in the droplet is similar but not identical to that of the gas-phase species, (b) the SF5+ photofragment velocity distribution is considerably slower upon droplet photoionization, and (c) fragmentation to SF4+ and SF3+ is much less than in the photoionization of bare SF6. From these measurements we obtain new insights into the mechanism of SF6 photoionization within the droplet and the cooling of the hot photofragment ions produced by dissociative photoionization.     

Rotational and vibrational dynamics of ethylene in helium nanodroplets

Rotationally resolved infrared spectra are reported for the asymmetric C-H stretching fundamental bands of C(2)H(4) in helium nanodroplets, as well as two weak combination bands. The J=2 rotor levels are strongly shifted from the energies estimated from a rigid rotor calculation and can be accounted for with two centrifugal distortion constants. The excited states of the three bands with B(3u) symmetry are strongly coupled in the gas phase and exhibit lifetimes >100 ps in helium, with the upper member of the polyad exhibiting the shortest lifetime. In contrast, the nu(9) band (B(2u) symmetry) exhibits very broad, homogeneously broadened line profiles (full width at half maximum approximately 0.5 cm(-1)) corresponding to an excited state lifetime of approximately 10 ps. This short lifetime is presumed to be due to an efficient, solvent mediated vibration-to-vibration relaxation process. In addition, the absence of transitions to the 2(21) and 2(20) rotor levels in the nu(9) band suggests they form rotational resonances with the elementary modes of helium, resulting in very short excited state lifetimes of less than 2 ps.     

Laser-induced fluorescence spectroscopy of 3,4,9,10-perylenetetracarboxylic-dianhydrid in helium nanodroplets

Electronic spectra of the S1<--S0 transition of the 3,4,9,10-perylenetetracarboxylic-dianhydrid (PTCDA) monomer isolated in superfluid helium nanodroplets have been measured by means of laser-induced fluorescence. The 0(0)(0) transition appears at 20,988 cm(-1) as the dominant line. We obtain clearly resolved the vibrational structure of the molecule. A comparison to Raman spectra of PTCDA films on metallic substrates and PTCDA crystals as well as with calculated frequencies provides the identification of the different modes. The enhanced resolution in the low temperature helium environment and the obtained line positions provide new information about structural properties of perylene derivatives.     

Core-shell effects in the ionization of doped helium nanodroplets

Core-shell particles with water clusters as the core and surrounded by an atomic or molecular shell have been synthesized for the first time by adding water and a co-dopant sequentially to helium nanodroplets. The co-dopants chosen for investigation were Ar, O(2), N(2), CO, CO(2), NO and C(6)D(6). These co-dopants have been used to investigate the effect of an outer shell on the ionization of the core material by charge transfer in helium nanodroplets. The specific aim was to determine how the identity of the shell material affects the fragmentation of water cluster ions, i.e. whether it helps to stabilize parent ion ((H(2)O)(n)(+)) formation or increases fragmentation (to form (H(2)O)(n)H(+)). N(2), O(2), CO(2) and C(6)D(6) all show a marked softening effect, which is consistent with the formation of a protective shell around the water cluster core. For CO and NO co-dopants, the response is complicated by secondary reactions which actually favour water cluster ion fragmentation for some water cluster sizes.