  The observed large variation in the abundance of deuterium (D) in the
interstellar medium (ISM) suggests that a significant fraction of D may be
depleted into polycyclic aromatic hydrocarbons (PAHs). Signatures of
deuteration of PAHs are expected to appear most clearly through C-D stretching
modes at 4.4--4.7 micron, whose strengths in emission spectra relative to those
of C-H stretching modes at 3.3--3.5 micron provide the relative abundance of D
to hydrogen (H) in PAHs once we have accurate relative band strengths of both
stretching modes. We report experimental results of the band strength of C-D
stretching modes relative to C-H. We employ a laboratory analogue of
interstellar carbonaceous dust, Quenched Carbonaceous Composite (QCC), and
synthesize deuterated QCC (D-QCC) by replacing the starting gas of CH$_4$ of
QCC by mixtures of CH$_4$ and CD$_4$ with various ratios. Infrared spectra of
D-QCC are taken to estimate the relative band strengths of the stretching
modes, while the D/H ratios in the D-QCC samples are measured with a nano-scale
secondary ion mass spectrometer (NanoSIMS). We obtain that the relative
strength of aromatic and aliphatic C-D to C-H stretches is 0.56 +/- 0.04 and
0.38 +/- 0.01 per D/H, respectively. The ratio for the aromatic stretches is in
good agreement with the results of theoretical calculations, while that of
aliphatic stretches is smaller than that of aromatic. The present results do
not significantly change the D/H ratios in the interstellar PAHs previously
estimated from observed spectra.
