Multibubble sonoluminescence as a tool to study the mechanism of formic acid sonolysis

• Sonoluminescence spectra of formic acid show C2 Swan band emission.
• C2 emission originates from CO formed during HCOOH sonolysis.
• Another sonochemical product, CO2, quenches strongly overall sonoluminescence.

Sonoluminescence spectra collected from 0.1 to 3.0 M aqueous solutions of formic acid sparged with argon show the OH(A2Σ+−X2Πi) and C2(d3Πg → a3Πu) emission bands and a broad continuum typical for multibubble sonoluminescence. The overall intensity of sonoluminescence and the sonochemical yield of HCOOH degradation vary in opposite directions: the sonoluminescence is quenched while the sonochemical yield increases with HCOOH concentration. By contrast, the concentration of formic acid has a relatively small effect on the intensity of C2 Swan band. It is concluded that C2 emission originates from CO produced by HCOOH degradation rather than from direct sonochemical degradation of HCOOH. The intensity of C2 band is much stronger at high ultrasonic frequency compared to 20 kHz ultrasound which is in line with higher yields of CO at high frequency. Another product of HCOOH sonolysis, carbon dioxide, strongly quenches sonoluminescence, most probably via collisional non-radiative mechanism.