Been reading up on intersystem crossing and have gotten rather confused by:

a photon possesses an angular momentum of h/2pi so it can couple to an electron spin and in principle induce any plausible spin transition between two spin states

This suggests to me that in a system with two coupled electron spins, the absorbed photon can provide angular momentum to allow a spin-flip between a singlet state to a triplet state (and conserve total angular momentum). i.e. an S(0) to T(1) transition is allowed if coupled to a photon.

I know this is not the case. Experimentally, the S(0) to T(1) transition has no oscillator strength and is not observed in an absorption spectra (even in molecules with no heavy-atoms for spin-orbit coupling). What am I neglecting the prevents the angular momentum of an absorbed photon coupling with an electron spin to allow a change in the spin?

Thanks

Been reading up on intersystem crossing and have gotten rather confused by:

a photon possesses an angular momentum of $h/2\pi$ so it can couple to an electron spin and in principle induce any plausible spin transition between two spin states

This suggests to me that in a system with two coupled electron spins, the absorbed photon can provide angular momentum to allow a spin-flip between a singlet state to a triplet state (and conserve total angular momentum). That is, an S  (0) to T  (1) transition is allowed if coupled to a photon.

I know this is not the case. Experimentally, the S (0) to T  (1) transition has no oscillator strength and is not observed in an absorption spectra (even in molecules with no heavy-atoms for spin-orbit coupling). What am I neglecting the prevents the angular momentum of an absorbed photon coupling with an electron spin to allow a change in the spin?

Been reading up on intersystem crossing and have gotten rather confused by:

a photon possess an angular momentum of h/2pi so it can couple to an electron spin and in principle induce any plausible spin transition between two spin states

This suggests to me that in a system with two coupled electron spins, the absorbed photon can provide angular momentum to allow a spin flip between a singlet state to a triplet state (and conserve total angular momentum). i.e. a S(0) to T(1) transition is allowed if coupled to a photon.

I know this is not the case. Experimentally, the S(0) to T(1) transition has no oscillator strength and is not observed in an absorption spectra (even in molecules with no heavy-atoms for spin-orbit coupling). What am I neglecting the prevents the angular momentum of an absorbed photon coupling with an electron spin to allow a change in spin?

Thanks

Been reading up on intersystem crossing and have gotten rather confused by:

a photon possesses an angular momentum of h/2pi so it can couple to an electron spin and in principle induce any plausible spin transition between two spin states

This suggests to me that in a system with two coupled electron spins, the absorbed photon can provide angular momentum to allow a spin-flip between a singlet state to a triplet state (and conserve total angular momentum). i.e. an S(0) to T(1) transition is allowed if coupled to a photon.

I know this is not the case. Experimentally, the S(0) to T(1) transition has no oscillator strength and is not observed in an absorption spectra (even in molecules with no heavy-atoms for spin-orbit coupling). What am I neglecting the prevents the angular momentum of an absorbed photon coupling with an electron spin to allow a change in the spin?

Thanks