Superhydrogenated polycyclic aromatic hydrocarbons ( PAHs ) may be present in H-rich and ultraviolet-poor benign regions .
The addition of excess H atoms to PAHs converts the aromatic bonds into aliphatic bonds , the strongest of which falls near 3.4 { \mu m } .
Therefore , superhydrogenated PAHs are often hypothesized as a carrier of the 3.4 { \mu m } emission feature which typically accompanies the stronger 3.3 { \mu m } aromatic C–H stretching feature .
To assess this hypothesis , we use density function theory to compute the infrared ( IR ) vibrational spectra of superhydrogenated PAHs and their ions of various sizes ( ranging from benzene , naphthalene to perylene and coronene ) and of various degrees of hydrogenation .
For each molecule , we derive the intrinsic oscillator strengths of the 3.3 { \mu m } aromatic C–H stretch ( A _ { 3.3 } ) and the 3.4 { \mu m } aliphatic C–H stretch ( A _ { 3.4 } ) .
By comparing the computationally-derived mean ratio of \langle A _ { 3.4 } / A _ { 3.3 } \rangle \approx 1.98 with the mean ratio of the observed intensities \langle I _ { 3.4 } / I _ { 3.3 } \rangle \approx 0.12 , we find that the degree of superhydrogenation — the fraction of carbon atoms attached with extra hydrogen atoms — is only \sim 2.2 % for neutral PAHs which predominantly emit the 3.3 and 3.4 { \mu m } features .
We also determine for each molecule the intrinsic band strengths of the 6.2 { \mu m } aromatic C–C stretch ( A _ { 6.2 } ) and the 6.85 { \mu m } aliphatic C–H deformation ( A _ { 6.85 } ) .
We derive the degree of superhydrogenation from the mean ratio of the observed intensities \langle I _ { 6.85 } / I _ { 6.2 } \rangle \lesssim 0.10 and \langle A _ { 6.85 } / A _ { 6.2 } \rangle \approx 1.53 for neutrals and \langle A _ { 6.85 } / A _ { 6.2 } \rangle \approx 0.56 for cations to be \lesssim 3.1 % for neutrals and \lesssim 8.6 % for cations .
We conclude that astrophysical PAHs are primarily aromatic and are only marginally superhydrogenated .