R. E. Miller

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

High-resolution near-infrared spectroscopy of water dimer

High‐resolution near‐infrared spectra are reported for all of the O–H stretch vibrational bands of the water dimer. The four O–H vibrations are characterized as essentially independent proton‐donor or proton‐acceptor motions. In addition to the rotational and vibrational information contained in these spectra, details are obtained concerning the internal tunneling dynamics in both the ground and excited vibrational states. These results show that for tunneling motions which involve the interchange of the proton donor and acceptor molecules, the associated frequencies decrease substantially due to vibrational excitation. The predissociation lifetimes for the various states of the dimer are determined from linewidth measurements. These results clearly show that the predissociation dynamics is strongly dependent on the tunneling states, as well as the Ka quantum number, indicating that the internal tunneling dynamics plays an important role in determining the dissociation rate in this complex.

The vibrational predissociation lifetime of the HF dimer upon exciting the ‘‘free‐H’’ stretching vibration

A computer controlled F‐center laser has been used, in conjunction with a molecular beam apparatus employing optothermal detection, to obtain an accurate measurement of the vibrational predissociation lifetime of the HF dimer following ν1 vibrational excitation. The lifetime obtained in this study is 24±2 ns. This is to be compared with a lifetime for ν2 excitation of 1.0±0.1 ns.

Structure and vibrational dynamics of the CO2 dimer from the sub‐Doppler infrared spectrum of the 2.7 μm Fermi diad

Sub‐Doppler infrared spectra of two Fermi resonance coupled bands of carbon dioxide dimer have been obtained at 3611.5 and 3713.9 cm−1 using an optothermal molecular beam color‐center laser spectrometer. The band origins for the complexes are red shifted by approximately 1 cm−1 from the corresponding ν1+ν3/2ν02+ν3 CO2 bands. The higher frequency band is perturbed while the lower frequency band appears free of extraneous perturbations as determined from a precision fit to a Watson asymmetric rotor Hamiltonian. This fit and the observed nuclear spin statistical weights reveal that the complex is planar with C2h symmetry. The C‐‐C separation and C‐‐C–O angle are determined to be 3.599(7) A and 58.2(8)°, respectively. The nearest neighbor O‐‐C distance is 3.14 A which is the same as that found in the crystal. From the centrifugal distortion analysis the weak bond stretching and symmetric bending frequencies are estimated to be 32(2) and 90(1) cm−1. No interconversion tunneling is observed.

The structure of the carbon dioxide dimer from near infrared spectroscopy

An F‐center laser–molecular beam spectrometer has been used to obtain a sub‐Doppler resolution infrared spectrum of the carbon dioxide dimer. The vibrational mode investigated in this study corresponds to the ν1+ν3 combination mode of the monomer located at 3716 cm−1. A qualitative assignment of the spectrum shows unambiguously that the equilibrium structure of the dimer is the slipped parallel, rather than the T‐shaped, geometry. The observed spectrum cannot be fit to within experimental error using conventional asymmetric rotor formalism. This may be due to a number of factors such as Fermi resonance between the upper state levels of the band and nearby levels of the dimer, such as seen in the monomer, or it could arise from tunneling effects arising from the two large amplitude motions in the dimer.

Photofragment angular distributions for HF dimer: Scalar J–J correlations in state‐to‐state photodissociation

Photofragment angular distributions have been measured for HF dimer which show resolved structure that can be assigned to individual fragment rotational channels. This data is used to establish intermolecular scalar correlations between the rotational states of the two HF fragments. The observed angular distributions are strongly dependent upon whether the ‘‘free’’ or ‘‘hydrogen bonded’’ HF stretch is initially excited. Since the infrared spectrum of the parent molecule is highly resolved, these results can be used to determine the relative state‐to‐state photodissociation cross sections. In addition, the zero point dissociation energy (D0 ) of the HF dimer is accurately determined.

SUB-DOPPLER INFRARED SPECTRUM OF THE CARBON DIOXIDE TRIMER

A spectrum of the carbon dioxide trimer van der Waals species has been recorded near 3614 cm−1 at sub‐Doppler resolution using an optothermal (bolometer‐detected) molecular‐beam color‐center laser spectrometer. A planar, cyclic structure with C3h symmetry has been determined for the complex with a carbon–carbon separation of 4.0382(3) A. The observed perpendicular band, corresponding to an in‐plane E′‐symmetry vibration of the trimer, has been attributed to a localized excitation of the 2ν02 +ν3 combination mode of a CO2 subunit by virtue of its small blue shift (∼0.98 cm−1) from that of the isolated monomer.

The argon–hydrogen fluoride binary complex: An example of a long lived metastable system

The optothermal laser–molecular beam method has been used to measure the infrared spectrum of Ar–HF [10°0←00°0]. The results show that the vibrational predissociation lifetime of this complex is greater than the flight time of the molecules from the laser crossing region to the bolometer detector. This gives a lower limit on the lifetime of 3×10−4 s! The upper vibrational state dipole moment has also been obtained for the complex (μ1=1.495 D) by carrying out infrared stark spectroscopy. This corresponds to a 12% increase in the dipole moment upon vibrational excitation. This change can be related to a stiffening of the van der Waals bond, and hence a reduction in the amplitude of the bending motion, in the vibrationally excited state.

Initial state effects in the vibrational predissociation of hydrogen fluoride dimer

The state‐to‐state vibrational predissociation dynamics of the hydrogen fluoride dimer has been investigated in detail using a newly developed instrument which gives both initial state selection and photofragment state determination. Results are reported for a wide variety of initial states associated with the ν1 and ν2 vibrations. The final state distributions universally indicate that the preferred dissociation channels correspond to the production of one HF fragment that is highly rotationally excited and another that is not. This is explained in terms of an impulsive dissociation mechanism which proceeds from a geometry close to that of the equilibrium structure of the dimer. We find that nearly degenerate initial states can have rather different final state distributions. In particular, there is a distinct difference between the upper and lower members of the tunneling doublet, which is most likely attributable to their related symmetries. The dissociation energy (D0) is determined to be 1062±1 cm−1.

The formation of cyclic water complexes by sequential ring insertion: Experiment and theory

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-poi...