Jeremy M. Merritt

Infrared spectroscopy of helium nanodroplets: novel methods for physics and chemistry

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 simply modify the molecular constants, while the molecular symmetry is maintained. In the case of vibrational spectroscopy, the perturbations due to the solvent are often so weak that the results can be compared directly with the theory for the corresponding isolated system. The growth of small clusters and nanoparticles in helium droplets is strongly influenced by the low temperature of the latter (0.37 K), often accentuating the effects of the long-range interactions between the constituent molecules. In many cases, these effects lead to the formation of exotic species that are difficult or impossible to make using more conventional techniques. Overall, helium nanodroplets act as a nearly ideal matrix for the synthesis and spectroscopic characterisation of these new and exotic species. Although there have been a number of previous reviews on helium nanodroplet spectroscopy, there are many important aspects of this emerging field that have yet to be suitably highlighted, making the present review timely. The goal here is to discuss some of the exciting new directions that are being explored using infrared laser spectroscopy as the probe. As noted above, the spectroscopy of impurities can provide interesting and new insights into the properties of liquid helium (including superfluidity, rotons, ripplons, etc.). Perhaps of even greater interest is the use of helium nanodroplets as nanocryostats for the growth of novel species, including those formed from metals, semiconductors, salts, biomolecules, free radicals, ions and hydrogen-bonding molecules. As we will demonstrate herein, helium nanodroplets provide considerable control over how these ‘nanomaterials’ are grown, opening up new possibilities for the formation and study of such species. Contents PAGE 1.  Introduction 16 2.  Experimental methods 19  2.1.  The droplets 19  2.2.  The pick-up technique 22  2.3.  Detection 24  2.4.  Infrared lasers 27  2.5.  Pendular state spectroscopy 28  2.6.  Vibrational Transition Moment Angles (VTMAs) 29  2.7.  Optically Selected Mass Spectrometry (OSMS) 32 3.  Molecular dynamics in helium 34  3.1.  Rotational dynamics 34   3.1.1.  Rotational relaxation rates 34   3.1.2.  The effective moment of inertia of solvated rotors 36   3.1.3.  Centrifugal distortion in helium solvated rotors 39  3.2.  Vibrational dynamics 41  3.3.  Photo-induced isomerisation 43 4.  Molecular clusters in helium nanodroplets 46  4.1.  The dynamics of cluster growth in helium nanodroplets 46  4.2.  Hydrogen clusters in helium nanodroplets 51  4.3.  Structural determination of metal atom cluster–adsorbate complexes 53  4.4.  Biomolecule and hydrated-biomolecule complexes 55  4.5.  Entrance and exit channel complexes – free radicals 57   4.5.1.  X–HY complexes 59   4.5.2.  Hydrogen abstraction reactions – organic radical chemistry 61   4.5.3.  Metal atom insertion reactions 63  4.6.  Salt clusters 65  4.7.  The structure and chemistry of semiconductor clusters 66 5.  Summary 67 Acknowledgements 68 References 69

Free radicals in superfluid liquid helium nanodroplets: A pyrolysis source for the production of propargyl radical

An effusive pyrolysis source is described for generating a continuous beam of radicals under conditions appropriate for the helium droplet pick-up method. Rotationally resolved spectra are reported for the ν1 vibrational mode of the propargyl radical in helium droplets at 3322.15 cm−1. Stark spectra are also recorded that allow for the first experimental determination of the permanent electric dipole moment of propargyl, namely, −0.150 and −0.148 D for ground and excited states, respectively, in good agreement with previously reported ab initio results of −0.14 D. The infrared spectrum of the ν1 mode of propargyl-bromide is also reported. The future application of these methods for the production of novel radical clusters is discussed.

Bonding in Beryllium Clusters

Beryllium clusters provide an ideal series for exploring the evolution from discrete molecules to the metallic state. The beryllium dimer has a formal bond order of zero, but the molecule is weakly bound. In contrast, bulk-phase beryllium is a hard metal with a high melting point. Theoretical calculations indicate that the bond energies increase dramatically for Be(n) clusters in the range n=2-6. A triplet ground state is found for n=6, indicating an early emergence of metallic properties. There is an extensive body of theoretical work on smaller Be(n) clusters, in part because this light element can be treated using high-level methods. However, the apparent simplicity of beryllium is deceptive, and the calculations have proved to be challenging owing to strong electron correlation and configuration interaction effects. Consequently, these clusters have become benchmark systems for the evaluation of a wide spectrum of quantum chemistry methods.

Infrared laser spectroscopy of CH3...HF in helium nanodroplets: The exit-channel complex of the F + CH4 reaction.

High-resolution infrared laser spectroscopy is used to study the CH3...HF and CD3...HF radical complexes, corresponding to the exit-channel complex in the F + CH4 --> HF + CH3 reaction. The complexes are formed in helium nanodroplets by sequential pickup of a methyl radical and a HF molecule. The rotationally resolved spectra presented here correspond to the fundamental v = 1 <-- 0 H-F vibrational band, the analysis of which reveals a complex with C(3v) symmetry. The vibrational band origin for the CH3...HF complex (3797.00 cm(-1)) is significantly redshifted from that of the HF monomer (3959.19 cm(-1)), consistent with the hydrogen-bonded structure predicted by theory [E. Ya. Misochko et al., J. Am. Chem. Soc. 117, 11997 (1995)] and suggested by previous matrix isolation experiments [M. E. Jacox, Chem. Phys. 42, 133 (1979)]. The permanent electric dipole moment of this complex is experimentally determined by Stark spectroscopy to be 2.4+/-0.3 D. The wide amplitude zero-point bending motion of this complex is revealed by the vibrational dependence of the A rotational constant. A sixfold reduction in the line broadening associated with the H-F vibrational mode is observed in going from CH3...HF to CD3...HF. The results suggest that fast relaxation in the former case results from near-resonant intermolecular vibration-vibration (V-V) energy transfer. Ab initio calculations are also reported (at the MP2 level) for the various stationary points on the F + CH4 surface, including geometry optimizations and vibrational frequency calculations for CH3...HF.

Infrared–infrared double resonance spectroscopy of cyanoacetylene in helium nanodroplets

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.

IR-IR double resonance spectroscopy in helium nanodroplets: Photo-induced isomerization

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.

Study of the CH3⋯H2O radical complex stabilized in helium nanodroplets

The weakly bound CH(3)H(2)O radical complex has been investigated by infrared laser spectroscopy. The complex is stabilized in helium nanodroplets and prepared by sequential pick up of a methyl radical and water molecule. Partially rotationally resolved spectra corresponding to the v = 1 <-- 0 excitation of the symmetric H(2)O stretching vibration within the complex show a significant red shift (25.06 cm(-1)) when compared with the symmetric stretch of H(2)O monomer, in agreement with the hydrogen bonded like structure derived by theory. Additional broad features were observed in the region predicted by theory for the antisymmetric stretch supporting our assignment. The B rotational constant is found to be 3.03 times smaller than predicted by ab initio calculations, with the reduction being attributed to the effects of helium solvation. The permanent electric dipole moment of the complex is experimentally determined to be 2.1 +/- 0.3 D using Stark spectroscopy. Ab initio calculations are also reported that provide support to the experimental results, as well as investigate the nature of large amplitude vibrational motion within the complex.

Spectroscopy, structure, and ionization energy of BeOBe.

Electronic transitions of BeOBe have been investigated using laser-induced fluorescence and resonance enhanced multiphoton ionization techniques in the 27000-33000 cm-1 range. Vibronic progressions observed in these spectra were assigned to the symmetric and antisymmetric stretching vibrations in the excited electronic state. The nuclear spin statistics of the ground state, observed in the intensity patterns of rotationally resolved spectra, confirmed that the molecule is symmetric (BeOBe) and has 1 Sigma(g)+ symmetry. Analysis of the rotational structure yielded a value of 1.396(3) A for the BeO bond length. Ground state vibrational frequencies were determined using stimulated emission pumping. Photoionization efficiency curves were recorded that yielded a value of 8.119(5) eV for the BeOBe ionization energy. Multireference electronic structure calculations have been used to predict molecular constants and explore the orbital compositions of the ground and excited states.

Experimental and theoretical studies of the electronic transitions of BeC

Electronic spectra for BeC have been recorded over the range 30,500-40,000 cm(-1). Laser ablation and jet-cooling techniques were used to obtain rotationally resolved data. The vibronic structure consists of a series of bands with erratic energy spacings. Two-color photoionization threshold measurements were used to show that the majority of these features originated from the ground state zero-point level. The rotational structures were consistent with the bands of (3)Π-X(3)Σ(-) transitions. Theoretical calculations indicate that the erratic vibronic structure results from strong interactions between the four lowest energy (3)Π states. Adiabatic potential energy curves were obtained from dynamically weighted MRCI calculations. Diabatic potentials and coupling matrix elements were then reconstructed from these results, and used to compute the vibronic energy levels for the four interacting (3)Π states. The predictions were sufficiently close to the observed structure to permit partial assignment of the spectra. Bands originating from the low-lying 1(5)Σ(-) state were also identified, yielding a (5)Σ(-) to X(3)Σ(-) energy interval of 2302 ± 80 cm(-1) and molecular constants for the 1(5)Π state. The ionization energy of BeC was found to be 70,779(40) cm(-1).

Experimental and theoretical investigations of rotational energy transfer in HBr + He collisions.

Rotational relaxation rates for HBr(v = 1) colliding with helium atoms at room temperature have been measured using a time-resolved optical-optical double resonance technique. Rotational state selective excitation of v = 1 for rotational levels in the range J = 1-9 was achieved by stimulated Raman pumping. The population decay in the prepared states and the transfer of population to nearby rotational states was monitored via 2 + 1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy using the g(3)Σ(-)-X(1)Σ(+) (0-1) band. Collision-induced population evolution for transfer events with |ΔJ| ≤ 8 was observed at pressures near 0.7 Torr. The experimental data were analyzed using fitting and scaling functions to generate state-to-state rotational energy transfer rate constant matrices. Total depopulation rate constants were found to be in the range (1.3 to 2.0) × 10(-10) cm(3) s(-1). As a test of current computational methods, state-to-state rotational energy transfer rate constants were calculated using ab initio theory. The total removal rate constants were in good agreement with the measured values, but the transfer probabilities for events with |ΔJ| ≥ 3 were underestimated. Inspection of the anisotropic characteristics of the potential energy surface did not yield an obvious explanation for the discrepancies, but it is most likely that the problem stems from inaccuracies in the potential surface.