Photoacoustic Spectroscopy

This sensitive technique for measuring small absorptions is mainly applied when tiny concentrations of molecular species have to be detected in the presence of other components at hight pressure. An example is the detection of spurious pollutant gases in the atmosphere in the IR spectral range.

The laser beam is sent through the absorber cell. If the laser is tuned to the absorbing molecular transition $\vert i> \rightarrow \vert k>$, part of the molecules in the lower level $\vert i>$ will be excited into the upper state $\vert k>$. By collisions with other atoms, or molecules in the cell these excited molecules may transfer their excitation energy $E_k-E_i$ to into translational, rotational, or vibrational energy of the collision partners. At thermal equilibrium, this energy is randomly distributed onto all degrees of freedom, causing an increase of thermal energy and with it a rise of temperature and pressure at a constant density in the cell.

When the laser beam is chopped at a frequency below 20 kHz, periodical pressure variations appear in the absorption cell which can be detected with a sensitive microphone placed inside the cell. The output signal $S$ of the microphone is proportional to the pressure change $\Delta p$ induced by the absorbed radiation power $\Delta W$:

$$S \propto \Delta W = N_i \sigma_{ik} l (1-\eta_k)P_L \Delta t,$$ (81)

where $N_i$ is the density of absorbing molecules in level $\vert i>$, $\sigma_{ik}$ is an absorption cross section, $l$ is an absorption path length, $\Delta t$ is a cycle period, $P_L$ is an incident laser power, and $\eta_{k}$ is a quantum efficiency which gives the ratio of emitted fluorescence energy to absorbed laser energy.

According to eq. (81) the optoacoustic signal decreases with increasing quantum efficiency because the fluorescence carries energy away without heating the gas, as long as the fluorescence light is not absorbed within the cell. Therefore, the optoacoustic method is particularly favorable to monitor vibrational spectra of molecules in the IR spectral region (because of the long lifetimes of excited vibrational states) and to detect small concentrations of molecules in the presence of other gases at higher pressure (because of the large collisional deactivation rate).

As an example, the optoacoustic method has been applied with great success to high-resolution spectroscopy of rotational-vibrational bands of numerous atmospheric molecules, like $C_2H_2$, where is spite of the small absorption coefficient a good signal-to-noise ration could be achieved.

Auf diesem Webangebot gilt die Datenschutzerklärung der TU Braunschweig mit Ausnahme der Abschnitte VI, VII und VIII.