Photodissociation Dynamics in Ordered Monolayers:  Physisorbed N2O4

The dynamics of photodissociation in physisorbed N2O4 were investigated by time-of-flight mass spectrometry. Ordered monolayers were prepared by adsorption of NO2 at 100 K on a water−ice surface of thickness ca. 30 monolayers held on a Cu(100) substrate. Irradiation of the sample surface at 355 nm resulted in exclusive desorption of NO2 with a most probable fragment translational energy of ca. 17 meV. Irradiation at 532 nm produced no desorbing species. The NO2 yield was linear in photon fluence, and the cross section for photodesorption was found to be (9 ± 2) × 10-18 cm2, similar to the gas phase absorption cross section at 355 nm. The angular distribution of the photodesorbed NO2 was peaked sharply in a direction around 10° from the normal, and no fragments were detected at angles greater than 30° from the normal. The photodissociation dynamics are dependent on the unique properties of the physisorbed system. The optical absorption in the adsorbate corresponds to the B̃B2u ← X̃1Ag transition in N2O4, as in the gas phase; however the subsequent dynamics in the excited state are markedly different from the gas phase counterpart. Despite the weak interaction with the substrate, significant energy transfer occurs in the ordered physisorbed monolayer to yield nascent NO2 with very low translational energy and a constrained angle of escape which is consistent with a high degree of adsorbate order and alignment.