Dissipative structures of diffuse molecular gas -: I.: Broad HCO+ (J=1-0) emission

Aims. Specific chemical signatures of the intermittent dissipation of turbulence were sought in diffuse molecular clouds. Methods. We observed HCO+(1-0) lines and the two lowest rotational transitions of (CO)-C-12 and (CO)-C-13 with an exceptional signal-to-noise ratio in the translucent environment of low-mass dense cores, where turbulence dissipation is expected to take place. Some of the observed positions belong to a new kind of small-scale structure identified in CO(1-0) maps of these environments as the locus of non-Gaussian velocity shears in the statistics of their turbulent velocity field, i.e. singular regions generated by the intermittent dissipation of turbulence. Results. We report the detection of broad HCO+(1-0) lines (10 mK < T*(A) < 0.5 K). We achieve the interpretation of ten HCO+ velocity components by conducting it in conjunction with that of the associated optically thin 13CO emission. The derived HCO+ column densities span a broad range, 10(11) < N(HCO+)/Delta v < 4 x 10(12) cm(-2)/km s(-1), and the inferred HCO+ abundances, 2 x 10(-10) < X(HCO+) < 10(-8), are more than one order of magnitude above those produced by steady-state chemistry in gas that is weakly shielded from UV photons, even at large densities. We compare our results with predictions of non-equilibrium chemistry, swiftly triggered in bursts of turbulence dissipation and followed by a slow thermal and chemical relaxation phase, assumed to be isobaric. The set of values derived from observations, i.e. large HCO+ abundances, temperatures in the range of 100-200 K, and densities in the range 100-10(3) cm(-3), unambiguously belongs to the relaxation phase. In contrast, the kinematic properties of the gas suggest that the observed HCO+ line emission results from a space-time average in the beam of the whole cycle followed by the gas and that the chemical enrichment is made at the expense of the non-thermal energy. Last, we show that the "warm chemistry" signature (i.e. large abundances of HCO+, CH+, H2O, and OH) acquired by the gas within a few hundred years, which is the duration of the impulsive chemical enrichment, is kept over more than a thousand years. During the relaxation phase, the H2O/OH abundance ratio stays close to the value measured in diffuse gas by the SWAS satellite, while the OH/HCO+ ratio increases by more than one order of magnitude.