COSMIC RAY-INDUCED CHEMISTRY TOWARD PERSEUS OB2

New measurements of absorption from interstellar OH at 3078 and 3081 A ngstrom are presented for three directions toward the Perseus OB2 asso ciation. The measured equivalent widths are in excellent agreement wit h previous determinations for o and zeta Per. Our measurements also pr ovide stringent upper limits for the line of sight to zeta Per. The av ailable data on lambda 3078 toward o and zeta Per with published equiv alent widths for the (0, 0) band of the D-X electronic transition near 1222 Angstrom are used to determine the oscillator strength for the D -X band; the value agrees with theory. The first ground-based detectio ns of absorption from interstellar Ti II lambda 3072 are discussed in an appendix. Cosmic-ray ionization followed by ion-molecule reactions is found to be the primary means of producing OH and HD in the environ ment probed by our observations. Our OH data are used in conjunction w ith a simple but adequate chemical model to obtain estimates of the co smic-ray ionization rate. These estimates then yield a value for the l ocal interstellar D/H ratio of (1.5 +/- 0.5) x 10(-5) from available o bservations of HD that is consistent with more direct measures. Furthe rmore, the derived rate allows us to place limits on the production of NH through gas-phase processes.