Biphotonic sequential

The study of short lived excited states is limited by the low concentra- lions in which they are created on excitation with normal light sources. The use of high intensity sources such as flash lamps with suitable flashing rates and laser sources have been helpful in this respect. Triplet-triplet absorption, absorption by excited singlet state to higher singlet state and Absorption by exciplexes (Section 6.6.1) can be effectively observed by sequential biphotonic processes. 

The ground-state depletion is more commonly observed in photochemical experiments such as flash photolysis than the simultaneous biphotonic absorption. Another common deviation from the Beer s law due to the sequential absorption of two photons will be discussed later in this section. 

In sequential, biphoton excitation, one photon is first absorbed to form an excited state if, during the lifetime of the excited state, a new photon is delivered and absorbed, a new higher energy excited state is formed ... 

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