ARTICLE pubs.acs.org/JPCA
Hydrogen Bonding in the Hexagonal Ice Surface Irene Li Barnett,† Henning Groenzin,‡ and Mary Jane Shultz§,* †
Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109, United States MB-technology GmbH, Kolumbusstrasse 2, 71063 Sindelfingen, Germany § Water and Surfaces Lab, Pearson Building, Chemistry Department, Tufts University, Medford, Massachusetts 02155, United States ‡
ABSTRACT: A recently developed technique in sum frequency generation spectroscopy, polarization angle null (or PAN-SFG), is applied to two orientations of the prism face of hexagonal ice. It is found that the vibrational modes of the surface are similar in different faces. As in the basal face, the prism face of ice contains five dominant resonances: 3096, 3146, 3205, 3253, and 3386 cm-1. On the basal face, the reddest resonance occurs at 3098 cm-1; within the bandwidth, the same as the prism face. On both the prism and basal faces, this mode contains a significant quadrupole component and is assigned to the bilayer stitching hydrogen bonds. The bluest of the resonances, 3386 cm-1, occurs slightly blue-shifted at 3393 cm-1 in the basal face. The prism face has two orientations: one with the optic or c axis in the input plane (the plane formed by the surface normal and the interrogating beam propagation) and one with the c axis perpendicular to the input plane. The 3386 cm-1 mode has significant intensity only with the c axis in the input plane. On the basis of these orientation characteristics, the 3386 cm-1 mode is assigned to double-donor molecules in either the top half bilayer or in the lower half bilayer. On the basis of frequency considerations, it is assigned to double-donor molecules in the top half bilayer. These are water molecules containing a nonbonded lone pair. In addition to identification of the components of the broad hydrogen-bonded region, PAN-SFG measures the tangential vs longitudinal content of the vibrational modes. In accord with previous suggestions, the lower frequency modes are predominantly tangential, whereas the higher frequency modes are mainly longitudinal. On the prism face, the 3386 cm-1 mode is entirely longitudinal.
’ INTRODUCTION Icy surfaces play a fundamental role in shaping the world. Ice is known to be involved transporting molecules throughout the Universe, in processing species involved in ozone destruction, and in generating lightning storms1-4 to name a few. Despite the importance of icy surfaces, there remain significant challenges for developing a molecular-level picture of the interfacial structure: a necessary starting point for generating a molecular-level understanding of interfacial interactions.6 The challenges arise in part due to the intrinsic disorder on the surface: The water molecules have near-tetrahedral bonding, but so-called proton disorder results in flexibility for dangling OH (d-OH) bonds and unsatisfied coordination - dangling oxygen (d-O) atoms - among the threecoordinate molecules on the surface. Distinguishing d-O and d-OH necessarily requires locating the hydrogen atoms. Many experimental techniques applied to ice7-13 positively locate the oxygen atoms but rely either on simulation10,13 or inference to find the hydrogen atoms. One method that is, in principle, capable of distinguishing these moieties is the vibrational spectroscopy, sum frequency generation (SFG). SFG has successfully probed numerous surfaces,14-27 including ice surfaces.5,28-32 Interpretation of the SFG data for ice, however, is hindered by lack of assignment of the hydrogen-bonded vibrational modes in the surface. The hydrogen bond region is particularly interesting due to the sensitivity of the OH vibrational frequency to the hydrogen bond environment, potentially containing a trove of information about interfacial interactions. This paper reports on application of a recently developed SFG polarization technique, polarization angle null (PAN-SFG), r 2010 American Chemical Society
coupled with examination of several faces of ice that provides experimental evidence for mode assignments. Efforts to assign the vibrational modes of ice have a long history. Whalley33 summarized the work prior to 1977 and argued for collective ν1 and ν3 vibrations. Later, Rice et. al34 argued against significant collective modes. Collective modes were revisited in a 1999 paper by Buch and Devlin.35 The fundamental shift in the latter consisted of viewing ice not as a collection of single water molecules with coupled symmetric and antisymmetric modes but rather as a collection of four-coordinate oxygen atoms with a degree of disorder leading to polarization of the tetrahedral unit cell. The result is a central infrared peak accompanied by modes at higher and lower frequencies. For thin films or particles, the bands at higher frequency are polarized perpendicular to the interface and were labeled as longitudinal/intermediate bands (L/I). Modes at lower frequency are polarized parallel to the interface and are termed transverse (T) bands. The present PAN-SFG data supports the transverse and longitudinal band assignments. The most stable form of ice under typical atmospheric conditions is hexagonal ice, Ih. Ice Ih consists of an hexagonal prism; the hexagonal or basal face is the one responsible for the characteristic six-sided shape of many snowflakes. The prism sides are termed the prism faces. This prism shape makes Ih ice birefingent. The optic or c axis is along the prism rotational axis. Special Issue: Victoria Buch Memorial Received: November 1, 2010 Revised: December 3, 2010 Published: December 28, 2010 6039
dx.doi.org/10.1021/jp110431j | J. Phys. Chem. A 2011, 115, 6039–6045
The Journal of Physical Chemistry A In principle, the basal face has 6-fold symmetry. However, single bilayer steps reduce that symmetry to 3-fold. Inevitable screw dislocations further reduce the symmetry so that in practice, the basal face is nearly isotropic. The prism face is not isotropic. Defining an input plane by the surface normal and the propagation direction of the laser beams used to probe the surface, a prism-face cut can be oriented with the c axis in the input plane or perpendicular to it. The hydrogenbond spectrum of the ice surface is sensitive to the orientation of the prism face. This paper discusses the sensitivity to orientation plus the polarization characteristics of the bands and suggests an assignment for two bands in the hydrogen bond region of the ice surface. SFG has previously been applied to the ice surface, focusing on characterizing surface premelting.28-30 This paper reports spectra for lower temperature and focuses on modes in the broad hydrogen-bonded region.
’ EXPERIMENTAL SECTION Spectroscopy. The basic theory of SFG has been described in several excellent reviews and tutorials,36-41 and the basic setup has been described previously,5,32,42,43 so only essential details are given here. Infrared (incident angle 60° measured from the normal, 1.1 mm diameter) and visible (532 nm, 20 ps, incident at 50°, 1.4 mm diameter) beams overlap spatially and temporally on the surface. For ice samples, it is imperative to limit the infrared power density as ice is an extremely strong infrared absorber. Typical pulse energies are 170 μJ for the visible and e40 μJ for the infrared. With e40 μJ infrared, the SF signal was linear in the infrared intensity, as required. The SF signal is filtered to reduce the residual visible, focused onto a 0.25 m monochromator and detected with a photomultiplier (Hamamatsu R4332). PAN-SFG44 is based on null angle techniques.45-53 PAN-SFG data is collected with the visible beam plane polarized þ45° (measured relative to the input plane, counterclockwise facing in the beam propagation direction) and the infrared p-polarized (that is, in the input plane). With this configuration, the SF beam can have any polarization. The SF polarization is measured by determining the output intensity as a function of the angle of a polarization analyzer placed in the detection arm. Within a dipole approximation, the principle of superposition of harmonic waves44 indicates that linearly polarized input beams produce a linearly polarized output beam: intensity falls as cosine squared from the polarization angle. (A more extensive discussion of the PAN technique can be found in ref 44.) Sample Preparation. Single crystal ice samples were grown from thoroughly degassed, 18 MΩ, UV-irradiated nanopure water in a modified Bridgeman apparatus. Ice crystals were characterized by polarized light, conoscopic imaging, and Formvar etching.32 (Formvar etching is described in references 54-56.) Typical surfaces used in these experiments had no grain boundaries and a screw dislocation density of
