Gigahertz Modulation of Femtosecond Time-Resolved Surface Sum-Frequency Generation Due to Acoustic Strain Pulses

Abstract

We study the properties of aqueous solution/air interfaces with time-resolved surface sum-frequency generation (SFG) spectroscopy. Using this technique, we monitor the change in SFG signal following the excitation of the water hydroxyl stretch vibrations with a strong ultrashort infrared pump pulse. We observe that, in addition to the excitation-induced changes in the signal resulting from vibrational excitation and relaxation on a time scale of picoseconds, the infrared excitation also induces a modulation of the SFG signal with a period of ∼180 ps. This modulation is caused by the interference between the SFG field directly radiated away from the air/liquid interface and the SFG field reflected or generated at the acoustic strain wavefront that is created by the infrared excitation pulse. The modulation is observed to be much stronger for aqueous salt solutions than for pure water, and the phase of the oscillations depends on the nature of the dissolved salt. This dependence of the phase can be explained from the differences in surface propensities of the ions of the different salts, giving rise to different net orientation of the interfacial water species.

Unified picture of vibrational relaxation of OH stretch at the air/water interface

“…2 in Supplementary Information). Such a long-lived component was also observed in a previous homodyne TR-VSFG study, and it was attributed to a water density gradient near the interface which is caused by the IR excitation 52 . Although the origin of this long-lived component is not very clear at the moment, we can consider that it does not represent any population dynamics of the excited-state HB OH because it remains even after the thermalization process completes.…”

Ions Speciation at the Water–Air Interface

“…The samples used in the TR-SFG data are the same as those used for the measurement of Figure b. The pure DCl and NaCl solutions exhibit the characteristic dynamics; the NaCl solution shows the characteristic time trace with a large oscillation, while DCl does not. The mixture solution of 0.5 M DCl + 0.5 M NaCl exhibits a very small oscillation, compared with the pure 1 M NaCl sample.…”

“…Thus, the long-lived oscillation serves as a reporter of the ion composition at the interface. 53 while DCl does not. The mixture solution of 0.5 M DCl + 0.5 M NaCl exhibits a very small oscillation, compared with the pure 1 M NaCl sample.…”

“…To probe the Na + signature, we carried out the TR-SFG measurement with homodyne detection. Hsieh et al reported long-lived oscillations (∼1 ns) in time-resolved IR pump/SFG probe signals for different ion solutions, 53 after the excess vibrational energy is fully relaxed (∼1 ps). This oscillation was assigned to the interference between the SFG signals generated at the water−air interface and that at the shock wave front.…”

Comprehensive Study of Thermal Transport and Coherent Acoustic-Phonon Wave Propagation in Thin Metal Film–Substrate by Applying Picosecond Laser Pump–Probe Method

“…The ultrafast laser pump–probe method has been applied to study the cross-plane thermal transport in thin film–substrate system. − ,− Actually, the temperature increase caused by ultrafast laser heating can also produce strain, that is, coherent lattice motion, which has been applied in ultrafast graphene ablation . Moreover, the ultrafast laser pump–probe technique is developed to study the generation, propagation, and attenuation of coherent acoustic phonons. − In this paper, the thermal transport and coherent acoustic-phonon wave propagation in thin Pt film-glass substrate system have been comprehensively studied by applying picosecond laser pump–probe method with front pump–front probe (FF), rear pump–front probe (RF), front pump–rear probe (FR), and rear pump–rear probe (RR) configurations. Significantly different time-dependent reflectance signals have been obtained in different configurations.…”