Biphotonic Ionization: A Flash Photoconductivity Study of TMPD in 3-Methylpentane Solutions

Flash photoconductivity studies are presented for a liquid organic solution. It was found that a dilute solution of N,N,N′,N′‐tetramethylpharaphenylenediamine in 3‐methylpentane at room temperature could be photoionized by light quanta with energies of the order of one‐half those required for gas‐phase ionization. This is fully reminiscent of the low‐energy photoionizations seen when this and similar solutions are cooled to a very rigid state at 77°K. These ionizations are now known to be biphotonic, and the possibility of a similar mechanism holding for the liquid, room‐temperature system is explored. A detailed kinetic analysis of one class of flash‐biphotonic phenomena is examined. The general category treated consists of all cases in which a one‐photon‐produced intermediate state is subsequently ionized by a second photon. The charge‐carrier density (and peak photocurrent) is proportional either to $${\left[{\displaystyle {\int }_0^T}\mathrm{I(t)dt}\right]}^2\mathrm{or\; to}{\displaystyle {\int }_0^T}{I}^2\mathrm{(t)dt}$$ , depending on whether the intermediate‐state lifetime is either long or short compared to the flash duration $T$ , respectively. Experimental studies verify proportionality of the peak photocurrent to both of these expressions, which, it happens, are equivalent under the conditions employed. The biphotonic character of the ionization is confirmed. A molecular‐weight study of the solute molecule verifies the monomer nature of the photosensitive species, and a linear concentration dependence of the photocurrent at low concentrations eliminates a bimolecular (solute–solute) mechanism in this instance. The sublinear dependence of photocurrent on concentration at higher concentrations is analyzed as a bimolecular quenching of the intermediate state. The range of possible intermediate‐state lifetimes obtained suggests (but does not prove) that, just as in the rigid solution at 77°K, the triplet state serves as the intermediate state in this fluid solution as well. The physical basis of the peak photocurrent as a meaningful parameter is discussed in an appendix.