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J. Phys. Chem. C 2007, 111, 9442-9447
Climbing the Vibrational Ladder To Probe the OH Stretch of HNO3 on Silica Andrew C. R. Pipino* Chemical Science and Technology Laboratory, National Institute of Standards and Technology (NIST), Gaithersburg, Maryland 20889
Marcin Michalski Lehrstuhl fu¨r Physikalische Chemie I, Technische UniVersita¨t Mu¨nchen, 5748 Garching, Germany ReceiVed: December 29, 2006; In Final Form: March 29, 2007
The first and second OH-stretching overtones of HNO3 adsorbed on atomically smooth amorphous SiO2 have been probed by evanescent wave cavity ring-down spectroscopy (EW-CRDS) using a broadband highfinesse total-internal-reflection-ring resonator. In contrast to the OH-stretching fundamental, the overtone spectra reveal relatively sharp features under conditions of extensive H-bonding. On the basis of a comparison with calculated vibrational frequencies, we find HNO3 exists predominantly as the HNO3-H2O complex on the surface under ambient conditions. Enabling unparalleled spectroscopic studies of monolayers, thin films, and nanoscale materials, the EW-CRDS technique yields polarized absolute absorption spectra for overtone and combination transitions at submonolayer coverage. The TIR-ring resonator in particular permits facile exploration of the visible and near-IR regions with a single optical configuration.
I. Introduction Detection of vibrational overtones and combination bands, or, more succinctly, multiquantum vibrational transitions, on planar surfaces has been achieved in relatively few investigations. Exceptionally high sensitivity is required to compensate for the typical order-of-magnitude reduction in absorption cross section incurred with each additional quantum involved. Binary transitions have been detected by Fourier transform infrared (FTIR) spectroscopy on metal1-6 and NaCl7 surfaces, while tertiary processes are typically inaccessible to conventional IR absorption techniques, even on planar metal surfaces. Some tertiary processes have been probed by electron energy loss spectroscopy8-12 (EELS), although UHV (ultra-high vacuum) conditions are required. Furthermore, detection of multiquantum transitions on oxide surfaces in particular has been achieved only by the use of high-surface-area porous forms to augment sensitivity, which produces a broad distribution of local environments.13-15 As the study of atomically smooth planar oxide surfaces has considerable current relevance,16-21 the development of techniques to enable insightful studies of surface structure and reactivity is needed. Despite challenges in detection, multiquantum vibrational transitions provide unique and important fundamental knowledge by probing the vibrational potential energy surface. For highly anharmonic vibrations such as those involving the ubiquitous H-X bond, an overtone series can enable bond length changes of ∼10-4 nm to be resolved as the vibrational manifold is climbed, even to relatively low excitation levels.22 The enhanced resolution is derived in part from greater localization of oscillator strength such that a specific bond can be identified as the chromophore. Moreover, the higher frequency associated with high-level excitations provides more rapid sampling of the local * Corresponding author. Current address: Applied Physics Department, Den Dolech 2, Technische Universiteit Eindhoven (TU/e), 5612 AZ Eindhoven, The Netherlands. E-mail:
[email protected].
chemical environment, thereby revealing structural details that would be otherwise averaged out. Although the increasing density of states with increasing energy would suggest greater opportunity for energy delocalization, intramolecular energy transfer in gas-phase species has been observed to occur through specific couplings that depend on the local conformation of the bond-chromophore.23-25 In the condensed phases, energy transfer is further modified by the local coordination environment, but the bond-chromophore perspective remains relevant.26 On surfaces, dipolar coupling can dramatically affect the line width of a vibrational fundamental, while an associated overtone or combination transition line width reflects only local surface heterogeneities. For example, the line width of C-O/Pt-C vibration-combination transition of CO adsorbed on Pt(111) has enabled dynamic dipole-coupling-induced line narrowing in the C-O fundamental to be identified.27 Similarly, local surface heterogeneities of CO adsorbed on NaCl have been probed through the first overtone.7 Although dipole-coupling must be considered in the determination of anharmonicities on surfaces28 and could induce delocalization for weakly anharmonic systems,5,8 small transition moments can be beneficial in this respect. Therefore, given the requisite sensitivity, the exploration of multiquantum vibrational transitions on surfaces will likely reveal new insights having both fundamental and technological relevance. Recently, Aarts et al.29 demonstrated detection of a 3-quantum vibration-combination transition on atomically smooth amorphous SiO2 with submonolayer sensitivity under ambient conditions. Specifically, the structure of a H2O monolayer H-bonded to hydroxylated SiO2 was revealed by probing the 2νOH + δOH transition in the near-IR, involving two quanta of OH-stretch (2νOH) and one of in-plane OH-bend (δOH). In contrast to fundamental OH-stretching spectra (νOH) of Hbonded systems, which are typically quite broad (>100 cm-1), the 2νOH + δOH transition revealed much structural detail. In particular, multiple fully resolved, highly polarized, and rela-
10.1021/jp0690244 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/09/2007
OH Stretch of HNO3 on Silica tively narrow (∼10 cm-1) spectral features were observed for both SiOH and H2O. Moreover, the H2O peaks showed sharpening and intensity saturation with increasing H2O coverage. Because the peak absorption cross section for the 2νOH + δOH transition is ∼10-22 cm2/molecule, a minimum detectable absorption of
