S0     Singlet ground state

S1     First excited singlet state; for C = O is often n pi*

S2     Second excited singlet state; for C = O is often pi pi*

a, a'  Represent processes of absorption of a photon and thus excitation to S1 or S2, respectively.

b  Radiationless internal conversion of S2 to S1(no emission of radiation). In a condensed phase this is a very rapid process so the lifetime in the S2 state is ca. 1 ps.

c  Reconversion to S0 from S1 with emission of radiation - called fluorescence. If fluorescence is the sole method of getting from S1 to S0, the lifetime of S1 would be ca. 106 - 103 ps.

d   Intersystem crossing from S1 to a vibrationally excited level of T1. This process is called a spin-forbidden transition.

e  Cascade from an upper to the lowest vibrational level of T1. This is a very rapid process as lifetimes in vibrationally excited levels are ca. 0.1 - 0.01 ps.

f  Conversion of T1 to S0 with emission of radiation, called phosphorescence, a spin forbidden process. For benzophenones this is also an symmetry forbidden process. The lifetime of T1 is ca.     10-3 - 10 s (or 109 - 1013 ps).

h, i  Radiationless internal conversion to ground state - a relatively slow deactivation process. Another route to the ground state is through external conversion ('quenching') that involves interaction with a second molecule, or quencher.

Since vibrations occur in ca. 0.1-0.01 ps, many vibrations are permitted to occur during the lifetimes of excited electronic states. Many excited states have energies that are sufficient to allow bond rupture and thus chemical change can occur at this point.

The spectroscopy of this experiment is concerned with (1) the identification and role of n pi* transitions compared with pi pi* and perhaps others such as charge transfer (CT ), and (2) the characteristics and role of the triplet state. The various types of spectroscopic measurements you will make are considered in order of their time sequence (Jablonski diagram ). It is helpful to become familiar with this diagram before making measurements in the lab.