Micron Band Research Articles

Variations in the Peak Position of the 6.2 μm Interstellar Emission Feature: A Tracer of N in the Interstellar Polycyclic Aromatic Hydrocarbon Population

This paper presents the results of an investigation of the molecular characteristics that underlie the observed peak position and profile of the nominal 6.2 micron interstellar emission band generally attributed to the CC stretching vibrations of polycyclic aromatic hydrocarbons (PAHs). It begins with a summary of recent experimental and theoretical studies ofthe spectroscopic properties of large (>30 carbon atoms) PAH cations as they relate to this aspect of the astrophysical problem. It then continues with an examination of the spectroscopic properties of a number of PAH variants within the context of the interstellar 6.2 micron emission, beginning with a class of compounds known as polycyclic aromatic nitrogen heterocycles (PANHs; PAHs with one or more nitrogen atoms substituted into their carbon skeleton). In this regard, we summarize the results of recent relevant experimental studies involving a limited set of small PANHs and their cations and then report the results of a comprehensive computational study that extends that work to larger PANH cations including many nitrogen-substituted variants of coronene(+) (C24H12(+)), ovalene(+) (C32H14(+)), circumcoronene(+) (C54H18(+)), and circum-circumcoronene(+) (C96H24(+)). Finally, we report the results of more focused computational studies of selected representatives from a number of other classes of PAH variants that share one or more of the key attributes of the PANH species studied. These alternative classes of PAH variants include (1) oxygen- and silicon-substituted PAH cations; (2) PAH-metal ion complexes (metallocenes) involving the cosmically abundant elements magnesium and iron; and (3) large, asymmetric PAH cations. Overall, the studies reported here demonstrate that increasing PAH size alone is insuEcient to account for the position of the shortest wavelength interstellar 6.2 micron emission bands, as had been suggested by earlier studies. On the other hand, this work reveals that substitution of one or more nitrogen atoms within the interior of the carbon skeleton of a PAH cation induces a significant blueshift in the position of the dominant CC stretching feature of these compounds that is suf6cient to account for the position of the interstellar bands. Subsequent studies of the effects of substitution by other heteroatoms (O and Si), metal ion complexation (Fe(+), Mg(+), and Mg(2+)), and molecular symmetry variation-all of which fail to reproduce the blueshift observed in the PANH cations-indicate that N appears to be unique in its ability to accommodate the position of the interstellar 6.2 micron bands while simultaneously satisfying the other constraints of the astrophysical problem. This result implies that the peak position of the interstellar feature near 6.2 micron traces the degree of nitrogen substitution in the population, that most of the PAHs responsible for the interstellar IR emission features incorporate nitrogen within their aromatic networks, and that a lower limit of 1%-2% of the cosmic nitrogen is sequestered within the interstellar PAH population. Finally, in view of the ubiquity and abundance of interstellar PAHs and the permanent dipoles and distinctive electronic structures of these nitrogen-substituted variants, this work impacts a wide range of observational phenomena outside of the infrared region of the spectrum including the forest of unidentified molecular rotational features and the anomalous Galactic foreground emission in the microwave, and the diffuse interstellar bands (DIBs) and other structure in the interstellar extinction curve in the ulhviolet/visible. These astrophysical ramifications are discussed, and the dipole moments and rotational constants are tabulated to facilitate further investigations of the astrophysical role of nitrogen-substituted aromatic compounds.

Formation and Evolution of Planetary Systems: Cold Outer Disks Associated with Sun‐like Stars

We present the discovery of debris systems around three solar mass stars based upon observations performed with the Spitzer Space Telescope as part of a Legacy Science Program, ``the Formation and Evolution of Planetary Systems'' (FEPS). We also confirm the presence of debris around two other stars. All the stars exhibit infrared emission in excess of the expected photospheres in the 70 micron band, but are consistent with photospheric emission at <= 33 micron. This restricts the maximum temperature of debris in equilibrium with the stellar radiation to T < 70 K. We find that these sources are relatively old in the FEPS sample, in the age range 0.7 - 3 Gyr. Based on models of the spectral energy distributions, we suggest that these debris systems represent materials generated by collisions of planetesimal belts. We speculate on the nature of these systems through comparisons to our own Kuiper Belt, and on the likely planet(s) responsible for stirring the system and ultimately releasing dust through collisions. We further report observations of a nearby star HD 13974 (d =11 pc) that is indistinguishable from a bare photosphere at both 24 micron and 70 micron. The observations place strong upper limits on the presence of any cold dust in this nearby system (L_IR/L_* < 10^{-5.2}).

Ices in the Edge‐on Disk CRBR 2422.8‐3423:SpitzerSpectroscopy and Monte Carlo Radiative Transfer Modeling

We present 5.2-37.2 micron spectroscopy of the edge-on circumstellar disk CRBR 2422.8-3423 obtained using the InfraRed Spectrograph (IRS) of the Spitzer Space Telescope. The IRS spectrum is combined with ground-based 3-5 micron spectroscopy to obtain a complete inventory of solid state material present along the line of sight toward the source. We model the object with a 2D axisymmetric (effectively 3D) Monte Carlo radiative transfer code. It is found that the model disk, assuming a standard flaring structure, is too warm to contain the very large observed column density of pure CO ice, but is possibly responsible for up to 50% of the water, CO2 and minor ice species. In particular the 6.85 micron band, tentatively due to NH4+, exhibits a prominent red wing, indicating a significant contribution from warm ice in the disk. It is argued that the pure CO ice is located in the dense core Oph-F in front of the source seen in the submillimeter imaging, with the CO gas in the core highly depleted. The model is used to predict which circumstances are most favourable for direct observations of ices in edge-on circumstellar disks. Ice bands will in general be deepest for inclinations similar to the disk opening angle, i.e. ~70 degrees. Due to the high optical depths of typical disk mid-planes, ice absorption bands will often probe warmer ice located in the upper layers of nearly edge-on disks. The ratios between different ice bands are found to vary by up to an order of magnitude depending on disk inclination due to radiative transfer effects caused by the 2D structure of the disk. Ratios between ice bands of the same species can therefore be used to constrain the location of the ices in a circumstellar disk. [Abstract abridged]

Extragalactic Source Counts in the Spitzer 24 Micron Band: What Do We Expect from ISOCAM 15 Micron Data and Models?

The comparison between the new Spitzer data at 24 μm and the previous ISOCAM data at 15 μm is key tool to understand galaxy properties and evolution in the infrared and to interpret the observed number counts, since the combination of Spitzer with the Infrared Space Observatory cosmological surveys provides for the first time the direct view of the universe in the infrared up to z 2. We present the prediction in the Spitzer 24 μm band of phenomenological model for galaxy evolution derived from the 15 μm data. Without any a posteriori update, the model predictions seem to agree well with the recently published 24 μm extragalactic source counts, suggesting that the peak in the 24 μm counts is dominated by starburst galaxies like those detected by ISOCAM at 15 μm but at higher redshifts (1 z 2 instead of 0.5 z 1.5).

Infrared Array Camera (IRAC) Observations of Planetary Nebulae

We present the initial results from the Infrared Array Camera (IRAC) imaging survey of planetary nebulae (PN). The IRAC colors of PN are red, especially when considering the 8.0 micron band. Emission in this band is likely due to contributions from two strong molecular hydrogen lines and an [Ar III] line in that bandpass. IRAC is sensitive to the emission in the halos as well as in the ionized regions that are optically bright. In NGC 246, we have observed an unexpected ring of emission in the 5.8 and 8.0 micron IRAC bands not seen previously at other wavelengths. In NGC 650 and NGC 3132, the 8.0 micron emission is at larger distances from the central star compared to the optical and other IRAC bands, possibly related to the molecular hydrogen emission in that band and the tendency for the molecular material to exist outside of the ionized zones. In the flocculi of the outer halo of NGC 6543, however, this trend is reversed, with the 8.0 micron emission bright on the inner edges of the structures. This may be related to the emission mechanism, where the molecular hydrogen is possibly excited in shocks in the NGC 6543 halo, whereas the emission is likely fluorescently excited in the UV fields near the central star.

Infrared Array Camera (IRAC) Imaging of the Lockman Hole

IRAC imaging of a 4'7x4'7 area in the Lockman Hole detected over 400 galaxies in the IRAC 3.6 micron and 4.5 micron bands, 120 in the 5.8 micron, and 80 in the 8 micron bandin 30 minutes of observing time. Color-color diagrams suggest that about half of these galaxies are at redshifts 0.6<z<1.3 with about a quarter at higher redshifts (z>1.3). We also detect IRAC counterparts for 6 of the 7 SCUBA sources and all 9 XMM sources in this area. The detection of the counterparts of the SCUBA sources and galaxies at z>1.3 demonstrates the ability of IRAC to probe the universe at very high redshifts.

Simulation of InAsSb/InGaAs Quantum Dots for Optical Device Applications

ABSTRACTSelf-assembled InAsSb/InGaAs quantum dots are candidates for optical detectors and emitters in the 2–5 micron band with a wide range of applications for atmospheric chemistry studies. It is known that while the energy band gap of unstrained bulk InAs1−xSbx is smallest for x=0.62, the biaxial strain for bulk InAs1−xSbx grown on In0.53Ga0.47As shifts the energy gap to higher energies and the smallest band gap is reached for x=0.51. The aim of the present study is to examine how the electronic confinement in the quantum dots modifies these simple considerations. We have calculated the electronic structure of lens shaped InAs1−xSbx quantum dots with diameter 37 nm and height 4 nm embedded in a In0.53Ga0.47As matrix of thickness 7 nm and lattice matched to an InP buffer. The relaxed atomic positions were determined by minimizing the elastic energy obtained from a valence force field description of the inter-atomic interaction. The electronic structure was calculated with an empirical tight binding approach. For Sb concentrations larger than x=0.5, it is found that the InSb/ In0.53Ga0.47As heterostructure becomes type II leading to no electron confined in the dot. It is also found that the energy gap decreases with increasing Sb content in contradiction with previous experimental results. A possible explanation is a significant variation is quantum dot size with Sb content.

Infrared camera onboard ASTRO-F

The design and the current development status of the Infrared Camera (IRC) onboard ASTRO-F is described. The IRC is designed for wide field imaging and slit-less spectroscopic observations at near- and mid-infrared wavelengths. The IRC consists of three channels, NIR, MIR-S and MIR-L, which cover wavelengths of 2–5, 5–12 and 12–26 micron, respectively. All of them consist of refractive optics. State-of-the-art large format array detectors are placed at the focal plane. Each channel has 10 × 10 arcmin wide field-of-view with diffraction-limited spatial resolution of 70 cm aperture of ASTRO-F telescope at wavelengths over 5 micron. We plan to have more than 7000 pointing observations dedicated for the IRC. The observational time of each pointing is limited to 500 seconds because of its low earth sun-synchronous polar orbit. The 5 sigma sensitivity of one pointing observation is estimated to be 2, 30 and 120 micro-Jy at 5, 9, 20 micron bands, respectively. Fabrication of the prototype model has been completed, and the performance tests are underway.

C–H Bond Formation in Carbon Grains by Exposure to Atomic Hydrogen: The Evolution of the Carrier of the Interstellar 3.4 Micron Band

We present the results of a systematic study on the interaction of nano-sized carbon grains with atomic hydrogen. The effects of H processing have been analyzed by infrared spectroscopy. The samples were irradiated with fluences ranging between 9.2 × 1016 and 1.3 × 1020 H atoms cm-2. Hydrogen atoms lead to the activation of the aliphatic C–H stretching and bending modes, whose intensity increases with exposure until saturation of hydrogenation. The plateau value of the absorption coefficient per unit mass of material at 3.4 μm is 1.6 × 103 cm2 g-1. The estimated cross section of C–H bond formation by H atoms for carbon particles is σf = (1.9 ± 0.5) × 10-18 cm2 per H atom, as derived from the behavior of the 3.4 μm band intensity as a function of the H atom fluence. We have found that the C–H bond formation depends on the structure of the carbon material that is exposed to atomic hydrogen. In view of the basic role of the hydrogenation of carbon particles by H atoms in interpreting the presence of the 3.4 μm band in the diffuse interstellar medium, the behavior of carbon materials under H processing becomes a fundamental criterion for constraining their applicability as analogs of the interstellar aliphatic component. The 3.4 μm band and the doublet at 6.86 and 7.26 μm of carbon particles processed by H atoms reproduce the peak positions and the relative intensities of those observed in the spectrum of interstellar dust toward the Galactic center. The estimation of the formation cross section of C–H bonds by H atoms, together with the previously determined destruction cross section by UV photons, allows a complete description of the evolution of the interstellar aliphatic carbon component due to UV and H processing. The conclusion of our analysis is that the interstellar component (i.e., the C–H bonds in the CH2 and CH3 groups) responsible for the 3.4 μm stretching band and the associated bending features at 6.85 and 7.25 μm is formed in the diffuse medium, since the carrier readily loses memory of its birthsite, wherever it is, because of interstellar processing, which determines a new equilibrium value for its degree of hydrogenation.

The 3 Micron Band of Crystalline Water Ice Irradiated by Lyman-αUV Photons: Comparison with Ion Irradiation Effects

We present the results of an experimental study of the structural effects induced by Lyman-α UV photons in crystalline water ice at low temperature carried out by using in situ infrared spectroscopy. We studied the evolution, under UV irradiation, of the 3 micron band profile. We found that an UV dose of few eV per water ice molecule is enough to fully amorphize the structure at the investigated temperature of 16 K. The evolution of the 3 micron feature during UV irradiation is compared with that observed for fast ion irradiation carried out in the same experimental set-up. Both photons and fast ions are present in space, they can induce variations in the profile of the 3 micron band of crystalline water ice. Thus these effects must be considered when one attempts to deduce, from the profile of the observed ice band, information on the temperature history of the responsible dust.

Glucose sensing based on interdigitated array microelectrode.

A micro glucose sensor consisting of an interdigitated array gold microelectrode was developed. The interdigitated array structure, which has 10 microns band width and 10 microns band gap, was fabricated in a small region (2.5 x 5 mm2) on a quartz substrate. Glucose oxidase was chemically fixed onto the electrode surface through self-assembled monolayer of 11-mercaptoundecanoic acid; ferroceneacetic acid was used as electron mediator. Electrochemical properties of the glucose oxidase-immobilized microelectrode were investigated by cyclic voltammogram measurements. Results confirmed that the reductive ferroceneacetic acid generated at counter electrode diffuses through a narrow band gap (10 microns) and can reach the working electrode surface.

Hydrogenation of polycyclic aromatic hydrocarbons as a factor affecting the cosmic 6.2 micron emission band

While many of the characteristics of the cosmic unidentified infrared (UIR) emission bands observed for interstellar and circumstellar sources within the Milky Way and other galaxies, can be best attributed to vibrational modes of the variants of the molecular family known as polycyclic aromatic hydrocarbons (PAH), there are open questions that need to be resolved. Among them is the observed strength of the 6.2 micron (1600 cm −1) band relative to other strong bands, and the generally low strength for measurements in the laboratory of the 1600 cm −1 skeletal vibration band of many specific neutral PAH molecules. Also, experiments involving laser excitation of some gas phase neutral PAH species while producing long lifetime state emission in the 3.3 micron (3000 cm −1) spectral region, do not result in significant 6.2 micron (1600 cm −1) emission. A potentially important variant of the neutral PAH species, namely hydrogenated-PAH (H N–PAH) which exhibit intriguing spectral correlation with interstellar and circumstellar infrared emission and the 2175 Å extinction feature, may be a factor affecting the strength of 6.2 micron emission. These species are hybrids of aromatic and cycloalkane structures. Laboratory infrared absorption spectroscopy augmented by density function theory (DFT) computations of selected partially hydrogenated-PAH molecules, demonstrates enhanced 6.2 micron (1600 cm −1) region skeletal vibration mode strength for these molecules relative to the normal PAH form. This along with other factors such as ionization or the incorporation of nitrogen or oxygen atoms could be a reason for the strength of the cosmic 6.2 micron (1600 cm −1) feature.

Detecting the first objects in the mid-infrared with the Next Generation Space Telescope

We calculate the expected Mid-IR molecular hydrogen line emission from the first objects in the universe. Due to their low masses, the stellar feedback from massive stars is able to blowaway their gas content and collect it into a cooling shell where H_2 rapidly forms and IR roto-vibrational (as for example the restframe 2.12 micron) lines carry away a large fraction (up to 10%) of the explosion energy. The fluxes from these sources are in the range 10^{-21}-10^{-17} erg/s/cm^2. The highest number counts are expected in the 20 micron band, where about 10^5 sources/deg^2 are predicted at the limiting flux of 3 \times 10^{-18} erg/s/cm^2. Among the planned observational facilities, we find that the best detection perspectives are offered by NGST, which should be able to reveal about 200 first objects in one hour observation time at its limiting flux in the above band. Therefore, Mid-IR instruments appear to represent perfect tools to trace star formation and stellar feedback in the high (z>5) redshift universe.

Laboratory investigation of the contribution of complex aromatic/aliphatic polycyclic hybrid molecular structures to interstellar ultraviolet extinction and infrared emission.

We have demonstrated by experiment that, in an energetic environment, a simple polycyclic aromatic hydrocarbon (PAH) such as naphthalene will undergo chemical reactions that produce a wide array of more complex species (an aggregate). For a stellar wind of a highly evolved star (post-asymptotic giant branch [post-AGB]), this process would be in addition to what is expected from reactions occurring under thermodynamic equilibrium. A surprising result of that work was that produced in substantial abundance are hydrogenated forms that are hybrids of polycyclic aromatic and polycyclic alkanes. Infrared spectroscopy described here reveals a spectral character for these materials that has much in common with that observed for the constituents of circumstellar clouds of post-AGB stars. It can be demonstrated that a methylene (-CH2-) substructure, as in cycloalkanes, is the likely carrier of the 6.9 microns band emission of dust that has recently been formed around IRAS 22272+5433, NGC 7027, and CPD -56 8032. Ultraviolet spectroscopy previously done with a lower limit of 190 nm had revealed that this molecular aggregate can contribute to the interstellar extinction feature at 2175 angstroms. We have now extended our UV spectroscopy of these materials to 110 nm by a vacuum ultraviolet technique. That work, described here, reveals new spectral characteristics and describes how material newly formed during the late stages of stellar evolution could have produced an extinction feature claimed to exist at 1700 angstroms in the spectrum of HD 145502 and also how the newly formed hydrocarbon material would be transformed/aged in the general interstellar environment. The contribution of this molecular aggregate to the rise in interstellar extinction at wavelengths below 1500 angstroms is also examined. The panspectral measurements of the materials produced in the laboratory, using plasmas of H, He, N, and O to convert the simple PAH naphthalene to an aggregate of complex species, provide insight into possible molecular structure details of newly formed hydrocarbon-rich interstellar dust and its transformation into aged material that becomes resident in the interstellar medium. Specifically the presence of naphthalene-like and butadiene-like conjugated structures as chromophores for the 2175 angstroms ultraviolet extinction feature is indicated.

Early results on comparison between cloud information by VIRS and rain information by PR/TMI

Using the infrared, passive microwave and radar data from the Tropical Rainfall Measuring Mission (TRMM), a correspondence is studied between cloud information and rainfall for three rain events in a preliminary study. Cloud information from the split window, 3.7 micron infrared spectrum and two visible channels from the Visible Infrared Scanner (VIRS) are compared with rainfall information from the Precipitation Radar (PR) and TRMM Microwave Imager (TMI). Optically thick clouds, as classified by the split window, reasonably correspond to the PR and TMI rainfalls. The smaller brightness temperature difference (BTD) between the 3.7 and 11 micron bands shows a better correspondence to the core of convection than the split window BTD. The radiance ratio of Ch1 Ch2 also shows a good correspondence to the PR and TMI rain area.

Activation of the 3.4 Micron Band in Carbon Grains by Exposure to Atomic Hydrogen

We present the results of an experiment aimed at studying the interaction of atomic hydrogen with nano-sized carbon grains. The effects induced by H processing have been studied by infrared spectroscopy. C–H stretching and bending modes are activated after H atom exposure. In particular, the 3.4 μm feature fits quite well to the absorption band observed in the diffuse interstellar medium toward the Galactic center as well as in the proto-planetary nebula CRL 618. The estimated efficiency of the process is such that one C–H bond is formed per 16 hydrogen atoms impinging on the sample. The results we obtained have important implications for the formation and evolution of carbon materials responsible for the 3.4 μm absorption band both in the diffuse interstellar medium and in the circumstellar outflows during the transition from the asymptotic giant branch to planetary nebula phases.

The interstellar 4.62 micron band.

We present new 4.5-5.1 micron (2210-1970 cm-1) spectra of embedded protostars, W33 A, AFGL 961 E, AFGL 2136, NGC 7538 IRS 9, and Mon R2 IRS 2, which contain a broad absorption feature located near 4.62 micron (2165 cm-1), commonly referred to in the literature as the "X-C triple bond N" band. The observed peak positions and widths of the interstellar band agree to within 2.5 cm-1 and 5 cm-1, respectively. The strengths of the interstellar 4.62 micrometers band and the ice absorption features in these spectra are not correlated, which suggests a diversity of environmental conditions for the ices we are observing. We explore several possible carriers of the interstellar band and review possible production pathways through far-ultraviolet photolysis (FUV), ion bombardment of interstellar ice analog mixtures, and acid-base reactions. Good fits to the interstellar spectra are obtained with an organic residue produced through ion bombardment of nitrogen-containing ices or with the OCN- ion produced either through acid-base reactions or FUV photolysis of NH3-containing ices.

Variations in the 3 micron spectrum across the Orion Bar: polycyclic aromatic hydrocarbons and related molecules.

Long-slit spectra across the Orion Bar reveal significant differences in the spatial behavior of the components of the 3 microns polycyclic aromatic hydrocarbon (PAH) spectrum. The strong PAH band at 3.29 microns generally decreases exponentially with distance from the ionization front into the molecular cloud (scale height approximately 12"), although excesses appear approximately 10" and 20" behind the ionization front, close to layers of H2 and CO emission, respectively. The 3.40 microns PAH feature separates into two components with very different spatial distributions. The main component (at 3.395 microns), along with the 3.51 microns band and the PAH plateau (3.3-3.6 microns), shows excess emission approximately 10" and approximately 20" behind the ionization front, stronger than the excesses in the 3.29 microns band. The extra component of the 3.40 microns band, which peaks at approximately 3.405 microns, has a spatial distribution very similar to the H2 emission. Aromatic C-H stretches in PAHs most likely produce the 3.29 microns feature. Aliphatic C-H stretches in either attached methyl side-groups or superhydrogenated PAHs, or perhaps both, could produce the complicated spectral and spatial structure at 3.40 microns.

The infrared spectrum of the Galactic center and the composition of interstellar dust.

We have obtained 5-8 micrometers spectra of the Galactic center from the Kuiper Airborne Observatory at resolving powers of approximately 50, approximately 150, and approximately 300. These spectra show absorption features at 5.5, 5.8, 6.1, and 6.8 micrometers. Together with previously observed features in the 3 micrometers region, these features are compared with laboratory spectra of candidate materials. The 3.0 and 6.1 micrometers features are due to the OH stretching and bending variations of H2O and are well fitted by water of hydration in silicates (e.g., talc). The 3.0 micrometer band is equally well fitted by ice mixtures containing 30% H2O, but such mixtures do not provide a good fit to the observed 6.1 micrometer band. The 3.4 and 6.8 micrometers features are identified with the CH stretching and deformation modes in CH2 and CH3 groups in saturated aliphatic hydrocarbons. The 6.1 micrometer band shows a short wavelength shoulder centered on 5.8 micrometer, attributed to carbonyl (C double bond O) groups in this interstellar hydrocarbon dust component. Finally, the narrow 5.5 micrometer feature is also attributed to carbonyl groups, but in the form of metal carbonyls [e.g., Fe(CO)4]. We have derived column densities and abundances along the line of sight toward the Galactic center for the various identified dust components. This analysis shows that hydrocarbon grains contain only 0.08 of the elemental abundance of C and contribute only a relatively minor fraction (0.1) of the total dust volume. Most of the interstellar dust volume is made up of silicates (approximately 0.6). Small graphite grains, responsible for the 2200 angstroms bump, account for 0.07 of the total dust volume. The remaining one-quarter of the interstellar dust volume consists of a material(s) without strong IR absorption features. Likely candidates include large graphite grains, diamonds, or amorphous carbon grains, which all have weak or no IR active modes. Finally, various models for the origin of the hydrocarbon dust component of the interstellar dust are discussed. All of them face some problems in explaining the observations, in particular, the absence of the spectroscopic signature of hydrocrbon grains in sources associated with molecular clouds.

Spatial Variation of the 3.29 and 3.40 Micron Emission Bands within Reflection Nebulae and the Photochemical Evolution of Methylated Polycyclic Aromatic Hydrocarbons

Spectra of 3 micrometers emission features have been obtained at several positions within the reflection nebulae NGC 1333 SVS3 and NGC 2023. Strong variations of the relative intensities of the 3.29 micrometers feature and its most prominent satellite band at 3.40 micrometers are found. It is shown that (i) the 3.40 micrometers band is too intense with respect to the 3.29 micrometers band at certain positions to arise from hot band emission alone, (ii) the 3.40 micrometers band can be reasonably well matched by new laboratory spectra of gas-phase polycyclic aromatic hydrocarbons (PAHs) with alkyl (-CH3) side groups, and (iii) the variations in the 3.40 micrometers to 3.29 micrometers band intensity ratios are consistent with the photochemical erosion of alkylated PAHs. We conclude that the 3.40 micrometers emission feature is attributable to -CH3 side groups on PAH molecules. We predict a value of 0.5 for the peak intensity ratio of the 3.40 and 3.29 micrometers emission bands from free PAHs in the diffuse interstellar medium, which would correspond to a proportion of one methyl group for four peripheral hydrogens. We also compare the 3 micrometers spectrum of the proto-planetary nebula IRAS 05341+0852 with the spectrum of the planetary nebula IRAS 21282+5050. We suggest that a photochemical evolution of the initial aliphatic and aromatic hydrocarbon mixture formed in the outflow is responsible for the changes observed in the 3 micrometers emission spectra of these objects.