We have developed a new technique for measuring the infrared absorption spectra of gases using Atomic Force Microscope microcantilevers.1 The photoacoustic system is demonstrated for a dilute acetylene/helium mixture by recording the acetylene ν13 infrared overtone transitions using a wavelength modulated tunable diode laser as the infrared light source. The technique presents significant advantages over existing methods in terms of size, simplicity, speed and insensitivity to ambient vibrations. The maximum achievable signal-to-noise for resonant and non-resonant photoacoustic excitation of the microcantilever is examined and is found to be limited by the microcantilever’s Brownian noise.
Section of the acetylene ν13 band recorded by lock-in detection of the AFM cantilever deflection signal as the laser wavelength was swept. The laser wavelength dither frequency corresponded to the cantilever resonance frequency (fmod=f0=5295Hz). Inset is an expansion of the P(7) line along with a fitted Gaussian derivative function (σD=0.026 cm-1). The sample was 50 mbar 2% C2H2 in He.
Working with Prof John Sader (Maths and Stats, UOM) we have analysed the AFM microcantilever amplitude response function with periodic excitation of the acetylene ν13 P(7) transition. Each trace represents the average of 50 frequency spectra, each taken with a 1 s acquisition time. For (a) the laser wavelength was modulated at fmod=3500 Hz. Note the fitted line (Eqn. 3) passing through the noise data points with fit parameters given in the figure. For (b) the laser wavelength was modulated at the cantilever resonance frequency (fmod=f0=5295 Hz). The gas sample was 50 mbar 2% C2H2 in He.
1. B.D. Adamson, J.E. Sader and E.J. Bieske, "Photoacoustic detection of gases using microcantilevers", Journal of Applied Physics, 106, 114510, 2009