Naphthalene and methyl-and halonaphthalene cations have been produced by one-and two-photon matrix photoionization techniques and trapped in solid argon for absorption spectroscopic study. Five transitions in the visible and ultraviolet regions have been assigned to the radical cations. The argon matrix absorption spectra are in agreement with photoelectron and photodissociation spectra, absorption spectra of the ions in Freon glasses, and simple HMO calculations. Substituent effects were observed for the origins of the five transitions. The repeating vibronic intervals in the red N+ transitions due to the C(9)-C(10) stretching fundamentals for this group of substituted naphthalene cations are 40-50 cm"1 above the Raman fundamentals for the molecules whereas the vibronic intervals due to ring deformation modes are up to 30 cm"1 below the appropriate Raman fundamentals, and vibronic intervals in the sharp UV band are 30-45 cm"1 below Raman fundamentals, which correlate with HMO it bond orders.

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