A new approach to determine the absolute photodissociation cross section of molecules in a cell

We demonstrate a method to determine the absolute value of the photodissociation cross section of a molecule of interest in a cell without knowing its absolute concentration. The target molecule can be either stable or unstable species. Proof-of-principle experiments have been demonstrated for the UV cross sections of CH2I2 and NO2. The relative concentrations of the target molecule were measured by monitoring its time-resolved UV-visible absorption spectra with a high-speed CMOS-camera spectrometer before and after its photodissociation by a pulsed 248- or 352-nm excimer laser beam. The absolute value of the photodissociation cross section can be deduced from the used laser fluence. The inhomogeneity of the photolysis laser beam has been found to be the major source of systematic error. However, this effect can be corrected by performing a calibration experiment on a molecule of which the photodissociation cross section is known. In some cases, post-photolysis reactions may affect the measurements. If the kinetics of the post-photolysis reactions is known, it is feasible to estimate their effect and retrieve the correct value of the photodissociation cross section. To our best knowledge, this is the first demonstration of such a method.

Automated summary

A method is demonstrated to determine the absolute value of the photodissociation cross section of a molecule in a cell without knowing its absolute concentration. The method involves monitoring the UV-visible absorption spectra of the target molecule before and after photodissociation by a pulsed laser beam, with corrections made for systematic errors caused by inhomogeneity of the photolysis laser beam. Post-photolysis reactions can affect measurements, but can be estimated and corrected if the kinetics of these reactions are known.