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===================================================================== ==> In this proposal, more time is being requested for LLITTLE.PROP_1 ===================================================================== The observed penetration of neutral carbon deep into molecular clouds poses problems both for standard models of PDR regions and for interstellar chemistry. We here propose to determine how the abundances of C+ and O vary with radius in a molecular cloud, G34.3+0.2, for which similar CI and O information of high quality already exists.This determination of the relative distribution of C+, C, O and CO will help substantially towards distinguishing between alternative models which have been proposed to account for the carbon penetration. These include: enhanced UV penetration into clouds due to clumping, generation of internal UV photons by cosmic rays exciting molecular hydrogen or by decay of dark matter neutrinos, oxygen depletion, a C/O ratio >1, turbulent mixing and gravitational contraction. Early observational studies of CI and CII indicated the widespread distribution of both species: it was established, for example, that the CI/CO ratio was high into depths where the optical extinction was 10 mag or greater, while standard PDR models predict that it should become very small for Av>4. It is difficult, however, to establish true, rather than 'column averaged' values for abundances deep in clouds. Recently, our study of the molecular cloud G34.3+0.2 suggested CI/CO declined from 0.4 near the perimeter to 0.04 as far in as Av=55 (Little et al 1994 MNRAS 271, 649). Our analysis was aided by the spherically symmetric structure of G34.3+0.2 and the conclusion was justified by very careful radiative transfer modelling of the angular structure and lineshapes of CI and several CO transitions. We propose to build on this effort by making observations across G34.3+0.2 in the CII 158mu, OI 63 and 146 mu lines to extend our determinations to the radial variation of C+ and O.