To perform absorption or emission spectra in gaussian 09, you need to use excited state calculation with proper method.

In gaussian 09, many kinds of method can be used to obtain spectrum such as TDDFT, CIS, CASSSCF, CASSPT2, EOMCCSD etc .

If your system is large, the TDDFT or CIS calculation is general option. The TDDFT gives better results than CIS.

To perform TDDFT, you need to mention TD option in route section [TD(NSTATE=X,singlet(triplet),root=1)]

For absorption spectra, geometry optimization of ground state need to be calculated then single point TDDFT should be performed based on Frank-condon principle.

For emission spectra, geometry optimization of excited state need to be calculated then single point TDDFT should be performed.

Hope this help.

Thanks! I would like to compare UPS measurements to these simulations. Would this be the emission spectra you mention above?

UPS data is different with absorption spectra or emission spectra.

Because UPS experiments processed based on Einstein photoelectric effect.

It indicates that the electron in valence shell will detached by proper energy. Using this results, it can determine orbital energy levels in bulk system.

If you wanna generate UPS data with gaussian 09 program, you should change the discrete orbital energy levels to continuous level.

First, you need to get orbital energy levels in ground state then you can change it to continuous level using Gausssum program.

You can find this program on web and it is free software.

Hope this help.