THz & PFI-ZEKE
CLUSTER SPECTROSCOPY
G.A. Blake Group
Divisions of Geological & Planetary Sciences,
Chemistry & Chemical Engineering
California Institute of Technology
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THz & PFI-ZEKE |
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Overview Intermolecular forces are ubiquitous in nature. From the icy mantles covering interstellar dust grains to the heart of living cells, van der Waals and hydrogen bonds play crucial roles in the regulation and evolution of both inorganic and living systems. Despite the explosive growth of new computational hardware and theoretical algorithms that will soon make it possible to examine such assemblages without approximation, current models of intermolecular forces are sometimes quite crude. The hydrogen bond is a highly revered dogma of modern science, transcending the boundaries of numerous disciplines. Typically, hydrogen bonds are assumed to involve essentially linear arrangements of donors, such as OH or NH groups with a proton acceptor, often a highly electronegative atom. It is now possible to quantitatively test these long-held views, and over the past decade the group has pioneered new techniques which will ultimately provide a complete understanding of van der Waals and hydrogen bonds. THz Spectroscopy of Hydrogen Bonded Clusters The first technique, developed with Prof. Rich Saykally and his group at UC Berkeley, couples oscillator and detector technology developed for heterodyne astronomy with molecular beam technology to create a truly general probe of intermolecular forces (for a review, see Saykally & Blake 1993). Specifically, microwave sidebands are placed onto an optically-pumped THz gas laser, and the resulting tunable sidebands are directed through a multi-pass cell surrounding a pulsed planar supersonic slit expansion (the figure below was kindly provided by Kun Liu). More recently, we have turned to the differency frequency generation of diode lasers in optical-heterodyne photomixers (to learn more about these devices, go to the light sources link below) in order to study the terahertz Vibration-Rotation-Tunneling Spectroscopy (THz-VRTS) of weakly bonded clusters.  The specific systems under investigation, most of which involve water, have been selected for their importance in cosmochemistry (H2O-CO, -N2, NH3-H2O) and biogeochemistry (C6H6-H2O, -NH3). Weak interactions are characterized by binding energies of at most a few kcal/mole and by intermolecular potential energy surfaces (IPS's) with a very rich and complex topology connected by barriers of at most a few hundred /cm. Rotational, tunneling, and intermolecular vibrational states can therefore become quite strongly mixed, hence the general term of vibration-rotation-tunneling (VRT) spectroscopy for the study of eigenvalues supported by an IPS. The VRT states in nearly all systems lie close to or above the tunneling barriers, and therefore sample {\bf large} regions of the potential surface. In addition, as they become spectroscopically observable the number, spacings, and intensities of the tunneling splittings are intimately related to the nature of the tunneling {\it paths} over the potential surface, as is pictorially illustrated below for the water trimer (image courtesy of the Saykally group).  In the case of the smallest clusters (that is, those with the least degrees of freedom), the observed data, which consists of rotational and vibrational term values along with the average projections of electric dipole and nuclear quadrupole moments onto the cluster inertial axes, may be directly inverted to generate quantitatively accurate global surfaces with modern supercomputers and fast numerical eigenvalue generators for the first time. Our approach has been to fit semi-empirical potentials whose gross shape is determined by high level ab initio calculations. Indeed, direct fits to experimental term values suffer from the fact that often the data is simply not complete enough to fully constrain the potential surface. By utilizing highly correlated wavefunctions in the ab initio calculations, sufficiently accurate parametric surfaces may be generated for species of chemical interest whose gross shape is placed on a firm theoretical footing without bias, yet whose parameters precisely fit the experimental results. In this way the simplicity and wide applicability of transferable potentials, essential to successful statistical studies of macroscopic systems, will become firmly rooted in the accurate generation of intermolecular forces. PFI-ZEKE Spectroscopy of Alkali-Solvent Cluseters While THz VRT studies form perhaps the most exacting means of investigating hydrogen bonded clusters, large concentrations must be generated. Thus, a number of intermolecular interactions, such as those between metal ions and polar molecules such as water, cannot yet be studied in the far-IR. In biological systems, many processes are based on the interaction between alkali ions and water or other molecules. To name but two, the Na+/K+ ATPase removes Na+ from and imports K+ into cells, while signal transmission in nerve cells also depends critically on the intermolecular interactions of Na+ and K+. The lack of high-resolution structural data on the membrane-bound ion channel proteins has led to a number of conflicting suggestions as to the mechanism by which selectivity is achieved. In the K+ binding site of dialkylglycine decarboxylase (DGD), for example, K+ is coordinated by six oxygen atoms in an octahedral arrangement that are provided by three carbonyl groups, a carboxyl group, a hydroxyl group, and a water molecule. When a Na+ ion occupies the site, the cavity shrinks to coordinate the smaller ion by five oxygens in a distorted trigonal bipyramidal geometry. Alternatively, the widespread occurrence of aromatic side groups along the acetylcholine receptor has led Dougherty and colleagues to suggest that cation-pi interactions are critical. Clearly, accurate potentials describing the interaction between alkali ions and water along with the groups that might line ion channels are required in order to model how protein pores achieve selectivity. We have therefore begun a program of measuring the VRT spectra of alkali-solvent clusters and their cations using PFI-ZEKE and REMPI techniques. Specifically, spectra of Na(H2O)n, Na(NH3)n, Na(C6H6)n clusters and their potassium counterparts have been collected with a crossed supersonic molecular beam chamber in which the alkali atoms are generated by laser vaporization (see figure at the top of the page). Photoionization mass spectra reveal clusters with up to 30-40 solvent molecules attached, and these studies will allow us to investigate the role of many body forces in the solvation of alkali and other metal cations, processes of tremendous chemical, biological, and environmental importance. In PFI-ZEKE spectroscopy, outlined in the figure below, optical pulses are used to excite molecules into Rydberg states lying just below the ionization limit for various rovibrational (or electronic) states of the cation. Under molecular beam conditions and with weak inhomogeneous electric fields, the high n, low l states pumped optically are converted into very long lived high n, high l states. With suitable temporal delays and the application of a small pulsed field, the Rydberg electrons can be selectively sampled. PIF-ZEKE thus permits photoelectron spectra to be acquired with nearly laser-limited resolution.  A sample spectrum for the Na(NH3) dimer is presented below, for which we have acquired single- and two-photon PFI-ZEKE spectra to provide partially rotationally resolved measurements of the bending and stretching states in both the neutral and ionic complexes for the first time, with ion-counting sensitivity. The one color spectra at left, for example, reveal short progressions in the cation stretching mode, as well as hot bands from the excited intermolecular stretching (nu3) and bending (nu6) states of the neutral complexes (T(vib) is ~100 K). The two photon spectra, taken through the Na(ND3) and Na(NH3) A state, can be used to isolate the neutral and cationic intermolecular modes. The complex lineshapes are the result of a combination of electron-ion scattering processes and the finite resolution of the OPOs, and we are theoretically studying them in collaboration with Prof. V. McKoy's group at Caltech.  Selected Publications "Tunable Far-Infrared Laser Spectrometers" Geoffrey A. Blake, Kenneth B. Laughlin, Ronald C. Cohen, Kerry L. Busarow, Duo H. Gwo, Charlie A. Schmuttenmaer, David W. Steyert, & Richard J. Saykally 1991, Rev. Sci. Instr. 62, 1693-1700. "Microwave and Tunable Far-Infrared Laser Spectroscopy of the Ammonia-Water Dimer" Paul A. Stockman, Roger E. Bumgarner, Sakae Suzuki, & Geoffrey A. Blake 1992, J. Chem. Phys. 96, 2496. "Hydrogen Bonding in the Benzene-Ammonia Dimer'' Dave Rodham, Sakae Suzuki, Richard Suenram, Fancis J. Lovas, Siddarth Dasgupta, William A. Goddard III, & Geoffrey A. Blake 1993, Nature 362, 735. "Molecular Interactions and Hydrogen Bond Tunneling Dynamics: Some New Perspectives'' Richard J. Saykally & Geoffrey A. Blake 1993, Science 259, 1570-1575. "Pseudorotation in the D2O Trimer'' Sakae Suzuki & Geoffrey A. Blake 1994, Chem. Phys. Lett., 229, 499. "ZEKE-PFI Spectroscopy of 1:1 Complexes of Sodium with Water and Ammonia" David Rodham & Geoffrey A. Blake 1997, Chem. Phys. Lett. 264, 522. "Microwave Spectroscopy of the Methanol-Water Dimer'' Paul Stockman, Richard Suenram, Francis J. Lovas, & Geoffrey A. Blake 1997, J. Chem. Phys. 107, 3782. "High Resolution PFI-ZEKE Photoelectron Spectroscopy of the Na(H2O) Complex'' Kwanghsi Wang, David A. Rodham, Vincent McKoy, & Geoffrey A. Blake 1998, J. Chem. Phys. 108, 4817. "Microwave and THz Spectroscopy" Geoffrey A. Blake 2001, Encylopedia of Chemical Physics & Physical Chemistry, J. Moore, N. Spencer, eds. (Institute of Physics Publ., Bristol), pp. 31-44. "Photoionization Spectroscopy of the Clusters of Potassium with H2O, NH3, and C6H6" Sheng Wu, Zulfikar Morbi, & Geoffrey A. Blake, J. Chem. Phys., in preparation. Links to Other Cluster Spectroscopy Groups Michael A. Duncan, Georgia William Klemperer, Harvard Kenneth R. Leopold, Minnesota Terry A. Miller, Ohio State David J. Nesbitt, Colorado Stewart E. Novick, Wesleyen (includes a WBC bibliography) Mitchio Okumura, Caltech Richard J. Saykally, UC Berkeley Timothy S. Zwier, Purdue Links to Cluster Reaction Dynamics Groups Hai-Lung Dai, Pennsylvania Marsha I. Lester, Pennsylvania W. Carl Lineberger, Colorado Verconica Vaida, Colorado Curt Wittig, USC Ahmed H. Zewail, Caltech Links to On-Line Journals Chemical Physics Letters The Journal of Chemical Physics The Journal of Physical Chemistry A (w/links to B) | Molecular Astrophysics | Atmospheric Chemistry | Light Sources | | G.A. Blake Home Page | GPS Home Page | Chemistry Home Page | | Astronomy Home Page | ESE Home Page | |