Quadrupole susceptibility automatic calculator in sum frequency generation spectroscopy incorporating magnetic contributions and refined local fields

Abstract

While sum frequency generation (SFG) spectroscopy is known to have acute interface selectivity owing to the dipole approximation, it has been a long-standing concern to understand the contribution of higher-order polarizations and/or bulk. Motivated by recent advances in microscopic theory of SFG including quadrupole, this work reports our recent development of the calculation tool of quadrupole susceptibility, namely, QSAC. QSAC aims at evaluating relative intensity of surface/bulk and dipole/quadrupole contributions for an arbitrary molecule and vibrational mode, in combination of quantum chemical calculations and molecular dynamics simulations. The present QSAC fully takes account of the generalized quadrupole, including electric quadrupole and magnetic dipole, and the local field effects on these polarizations as well as the electric dipole. We applied it to methyl C–H stretching and carbonyl C=O stretching modes and thereby demonstrated the significantly improved performance of the QSAC program to evaluate the contribution of quadrupole. We found that the accurate treatment of local field gradient is important in evaluating the quadrupolar susceptibility of the interface χIQ and that the magnetic contribution is generally comparable with that of electric quadrupole and thus substantial in the quadrupolar susceptibilities. The revised QSAC provides a reasonably reliable estimate of the bulk contribution in SFG spectra.