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The observed large variation in the abundance of deuterium (D) in theinterstellar medium (ISM) suggests that a significant fraction of D may bedepleted into polycyclic aromatic hydrocarbons (PAHs). Signatures ofdeuteration of PAHs are expected to appear most clearly through C-D stretchingmodes at 4.4--4.7 micron, whose strengths in emission spectra relative to thoseof C-H stretching modes at 3.3--3.5 micron provide the relative abundance of Dto hydrogen (H) in PAHs once we have accurate relative band strengths of bothstretching modes. We report experimental results of the band strength of C-Dstretching modes relative to C-H. We employ a laboratory analogue ofinterstellar carbonaceous dust, Quenched Carbonaceous Composite (QCC), andsynthesize deuterated QCC (D-QCC) by replacing the starting gas of CH$_4$ ofQCC by mixtures of CH$_4$ and CD$_4$ with various ratios. Infrared spectra ofD-QCC are taken to estimate the relative band strengths of the stretchingmodes, while the D/H ratios in the D-QCC samples are measured with a nano-scalesecondary ion mass spectrometer (NanoSIMS). We obtain that the relativestrength of aromatic and aliphatic C-D to C-H stretches is 0.56 +/- 0.04 and0.38 +/- 0.01 per D/H, respectively. The ratio for the aromatic stretches is ingood agreement with the results of theoretical calculations, while that ofaliphatic stretches is smaller than that of aromatic. The present results donot significantly change the D/H ratios in the interstellar PAHs previouslyestimated from observed spectra.

Laboratory measurements of stretching band strengths of deuterated Quenched Carbonaceous Composites (D-QCC)