Single Photon Infrared Emission Spectroscopy:  A Study of IR Emission from UV Laser Excited PAHs between 3 and 15 μm

Abstract

Single-photon infrared emission spectroscopy (SPIRES) has been used to measure emission spectra from polycyclic aromatic hydrocarbons (PAHs). A supersonic free-jet expansion has been used to provide emission spectra of rotationally cold and vibrationally excited naphthalene and benzene. Under these conditions, the observed width of the 3.3-micrometers (C-H stretch) band resembles the bandwidths observed in experiments in which emission is observed from naphthalene with higher rotational energy. To obtain complete coverage of IR wavelengths relevant to the unidentified infrared bands (UIRs), UV laser-induced desorption was used to generate gas-phase highly excited PAHs. Lorentzian band shapes were convoluted with the monochromator-slit function in order to determine the widths of PAH emission bands under astrophysically relevant conditions. Bandwidths were also extracted from bands consisting of multiple normal modes blended together. These parameters are grouped according to the functional groups mostly involved in the vibration, and mean bandwidths are obtained. These bandwidths are larger than the widths of the corresponding UIR bands. However, when the comparison is limited to the largest PAHs studied, the bandwidths are slightly smaller than the corresponding UIR bands. These parameters can be used to model emission spectra from PAH cations and cations of larger PAHs, which are better candidate carriers of the UIRs.

Peripherally Hydrogenated Neutral Polycyclic Aromatic Hydrocarbons as Carriers of the 3 Micron Interstellar Infrared Emission Complex: Results from Single‐Photon Infrared Emission Spectroscopy

“…The Berkeley Single Photon InfraRed Emission Spectrometer (SPIRES) is diagramed in Figure 1 and has been described in detail before (Cook et al 1998). The Ðve samples examined in this study, 2-methylnaphthalene (98%), 2-methylphenanthrene (98%), 1,2,3,6,7,8-hexahydropyrene (98%), 9,10-dihydrophenanthrene (94%), and acenaphthene (97%), were purchased from Aldrich and were used without further puriÐcation.…”

“…Complex No PAH emission spectrum has been able to reproduce the UIR spectrum with respect to either band positions or relative intensities (Cook et al 1998). Most notably, the 3.3 km band is far too intense in neutral PAHs, wherein it dominates the spectrum of most samples studied (Cook et al 1998).…”

“…Note the plateau emission not accounted for by the sum of the aliphatic CwH features red of 3.3 km, and (c) the Ðt residuals. modes (combination or sequence bands ; Cook et al 1998). However, other processes cannot be ruled out.…”

“…Most notably, the 3.3 km band is far too intense in neutral PAHs, wherein it dominates the spectrum of most samples studied (Cook et al 1998). Ionized PAHs are currently thought to better match the UIR band relative intensities, and hence have become the favored candidates for the UIR carriers.…”

“…However, because Gomez all aromatic molecules exhibit this feature, it is not a deÐni-tive diagnostic for assignment of the UIRs to a particular carrier. The extension of gas-phase emission studies deeper into the IR made possible by our development of the SPIRES technique (Schlemmer et al 1994 ;Cook et al 1996), has shown that neutral PAHs as large as coronene fail to reproduce the IR emissions in both band positions and relative intensities (Cook et al 1998). …”