Multiphoton ionization and photodissociation of (NO)mArn clusters

Picosecond multiphoton ionization of (NO)_mAr_n clusters produced in a supersonic expansion of NO/Ar gas mixtures has been studied using time-of-flight mass spectrometry. Two-photon ionization with 266 nm photons show that dilute gas mixtures (1% NO/Ar) yield clusters limited to m≤7, but with as many as 37 argon atoms. Magic numbers are observed for NO⁺Ar₁₂, NO⁺Ar₁₈, (NO) ₂ ⁺ Ar₁₇, NO⁺Ar₂₂, and (NO) ₂ ⁺ Ar₂₁ and are understood in terms of solvation of the NO⁺ and (NO) ₂ ⁺ by argon in icosahedral arrangements. Four-photon ionization with 532 nm light produces dissociation of the clusters to yield only NO⁺Ar_n with n up to 54. This distribution exhibits an additional magic number at n=54, consistent with the completion of a second solvation sphere about the NO⁺. The known wavelength dependence for photodissociation of (NO) ₂ ⁺ and (NO) ₃ ⁺ and comparison of MPI spectra obtained with 266, 355, and 532 nm light indicate that the dissociation is occurring in the cluster ions.