  Single-charge tunneling is a decoherence mechanism affecting superconducting
qubits, yet the origin of excess quasiparticle excitations (QPs) responsible
for this tunneling in superconducting devices is not fully understood. We
measure the flux dependence of charge-parity (or simply, ``parity'') switching
in an offset-charge-sensitive transmon qubit to identify the contributions of
photon-assisted parity switching and QP generation to the overall
parity-switching rate. The parity-switching rate exhibits a
qubit-state-dependent peak in the flux dependence, indicating a cold
distribution of excess QPs which are predominantly trapped in the low-gap film
of the device. Moreover, we find that the photon-assisted process contributes
significantly to both parity switching and the generation of excess QPs by
fitting to a model that self-consistently incorporates photon-assisted parity
switching as well as inter-film QP dynamics.
