Rethinking Vibrational Stark Spectroscopy: Peak Shifts, Line Widths, and the Role of Non-Stark Solvent Coupling

Abstract

A vibration's transition frequency is partly determined by the first-order Stark effect, which accounts for the electric field experienced by the mode. Using ultrafast infrared pump−probe and FT-IR spectroscopies, we characterized both the 0 → 1 and 1 → 2 vibrational transitions' field-dependent peak positions and line widths of the CN stretching mode of benzonitrile (BZN) and phenyl selenocyanate (PhSeCN) in ten solvents. We present a theoretical model that decomposes the observed line width into a field-dependent Stark contribution and a field-independent non-Stark solvent coupling contribution (NSC). The model demonstrates that the fielddependent peak position is independent of the line width, even when the NSC dominates the latter. Experiments show that when the Stark tuning rate is large compared to the NSC (PhSeCN), the line width has a field dependence, albeit with major NSC-induced excursions from linearity. When the Stark tuning rate is small relative to the NSC (BZN), the line width is field-independent. BZN's line widths are substantially larger for the 1 → 2 transition, indicating a 1 → 2 transition enhancement of the NSC. Additionally, we examine, theoretically and experimentally, the difference in the 0 → 1 and 1 → 2 transitions' Stark tuning rates. Second-order perturbation theory combined with density functional theory explain the difference and show that the 1 → 2 transition's Stark tuning rate is ∼10% larger. The Stark tuning rate of PhSeCN is larger than BZN's for both transitions, consistent with the theoretical calculations. This study provides new insights into vibrational line shape components and a more general understanding of the vibrational response to external electric fields.

Restricted Orientation Anisotropy Method for FVE Radii Characterization: Confirmed and Refined via the Study of Six Vibrational Probes

“…The spectra deviate from the Gaussian fit on the low-frequency side. The Stark effect on the frequency of the CN stretch dominates the inhomogeneous widths and the line shapes . The deviation from Gaussian line shapes is a manifestation of the non-Gaussian distribution of electric fields in the FVEs, which gives rise to non-Gaussian frequency dependence.…”

Structural Dynamics of a Novel Pseudohalide Perovskite Cs2Pb(SeCN)2Br2 Investigated with Nonlinear Infrared Spectroscopy

“…When the structure of the lattice in the vicinity of an SCN – changes, the interactions with the anion will change. The CN stretch has substantial Stark coupling that contributes to the inhomogeneous linewidth . Small changes in the lattice configuration can produce changes in the electric field along the CN bond vector.…”

“…The CN stretch has substantial Stark coupling that contributes to the inhomogeneous linewidth. 45 Small changes in the lattice configuration can produce changes in the electric field along the CN bond vector. Such changes will cause the vibrational frequency to change, giving rise to spectral diffusion, that is, sampling of different frequencies within the absorption spectrum.…”

Influence of Internal Bond Rotation on Ultrafast IR Anisotropy Measurements and the Internal Rotational Potential

“…Combination band peaks in the pump–probe spectrum of pClBZN, which arise from the coupling of the CN stretch to ring modes, grow in with the vibrational lifetime. These peaks overlap with the CN 0–1 absorption, preventing the accurate collection of the anisotropy data by measurements on the 0–1 transition . Therefore, the pClBZN anisotropy data were taken on the center frequency of the 1–2 transition at 2207.7 cm –1 .…”

“…These peaks overlap with the CN 0−1 absorption, preventing the accurate collection of the anisotropy data by measurements on the 0−1 transition. 25 Therefore, the pClBZN anisotropy data were taken on the center frequency of the 1−2 transition at 2207.7 cm −1 . In general, in a pump−probe experiment, measurements on the 0−1 and 1−2 transitions yield the same results.…”