Measurement of IR Multi Photon Absorption by Polyatomic Molecules Using the Photoacoustic Technique

Abstract

The single pulse photoacoustic technique for measuring infrared multiphoton absorption by polyatomic molecules is described in detail for the first time. The interaction volume of interest is the focal region (Fig. 1) in which the laser intensity can be very high to promote multiphoton processes. Compared to classical transmission technique with a parallel laser beam of very high intensity (practically very difficult), the present technique is simple but powerful. The microphone detects the energy released from the focal volume in front of it. As long as the microphone is closer to the focal volume than it is to the entrance and exit windows, the signal from the focal region will arrive at the microphone earlier than any other acoustic signal created in the cell. Fig. 2a shows two typical oscilloscope traces at two different time scales of the microphone signal by focusing a 10.6 µ TEA-CO2 laser pulse (~ 1 mJ/cm2) into 3 torr (1 torr = 0.133 kPa) SF6 through a 25.4 cm focal length lens. The focal region in front of the 1.27 cm diameter microphone can be approximated as a cylinder of uniform intensity and length ℓm because of the long focal length. The peak value of the first pulse is proportional to the energy absorbed by the molecules in the focal volume. Three questions are asked. (1) Why is the first peak proportional to the energy absorbed? (2) What are the other oscillations? (3) What is the pressure range for easy application of the technique? The answers are given below.