Ionized polycyclic aromatic hydrocarbon molecules ...

Ionized polycyclic aromatic hydrocarbon molecules and the interstellar extinction curve Wei Lee and Thomas J. Wdowiak

Citation: AIP Conference Proceedings 312, 675 (1994); doi: 10.1063/1.46592 View online: http://dx.doi.org/10.1063/1.46592 View Table of Contents: http://aip.scitation.org/toc/apc/312/1 Published by the American Institute of Physics

IONIZED POLYCYCLIC AROMATIC HYDROCARBON MOLECULES AND THE INTERSTELLAR EXTINCTION CURVE Wei Lee and Thomas J. Wdowiak Department of Physics University of Alabama at Birmingham, Birmingham, AL 35294, USA ABSTRACT Electronic absorption measurements have been carded out on both the neutral and cationic forms of several individual compact and non-compact PAH species in the wavelength range of 190--820 nm. The pronounced decrease in the strength of the strong near-UV absorption bands of the neutral species upon ionization by gamma radiation is indicated by the laboratory spectra. This provides persuasive experimental evidence for resolution of the conflict between the PAH-UIR hypothesis and the fact that expected features, which are characteristic of neutral aromatics in near-UV, are not observed in the interstellar extinction curve. INTRODUCTION

The principal infrared interstellar emission features, known as the unidentified infrared (UIR) bands at 3.3, 6.2, 7.7, 8.6, and 11.3 pm (3040, 1615, 1310, 1150, and 885 cm"), have been observed in the planetary nebulae, bipolar nebulae, reflection nebulae, H II regions, Herbig Ae stars, Wolf-Rayet stars, novae, the plane of the Milky Way, and starburst galaxies.' The fact that these five features are usually observed as a "family" and are associated with such a wide variety of celestial objects implies that there exists a relatively specific form of matter which is widely distributed in the interstellar medium (ISM) as a major constituent. Attempts to identify these emission features have led to generally accepted hypotheses that radiative relaxation of vibrationally excited aromatic hydrocarbons (PAHs) is involved. The original suggestion that the bands are associated with aromatic species embedded in small amorphous carbon particles2 was later challenged by the hypothesiss that they originate from vapor-phase, neutral polycyclic aromatic hydrocarbons. Allamandola, Tielens & Barker4 independently argued that because a large fraction of the PAHs are expected to be ionized, the PAH cations are mostly responsible for the bands. PAH species (ionized and neutral) are attractive as an interstellar component because their aromatic structure provides the level of stability necessary for long lifetime in the interstellar UV radiation and shock environment, and also allows for the conversion of the absorbed UV energy into discrete IR emission bands through the mechanism of IR fluorescence.5 The PAH-UIR hypothesis has long been investigated via a number of laboratory studies involved with neutral species because of the inherent difficulties in handling ionized molecules in the laboratory. Theoretical studies have for the most part until recently focused on neutral PAH molecules. It has been thought that the vibration modes of neutral PAHs and those of ionized species should not be much 9 1994 American Institute of Physics

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different. While most laboratory studies were done at room temperature, recent study of the temperature-dependent aspects~ of PAH molecules in the vapor phase and condensed state has shown that the C--H stretch feature of compact PAH species at elevated temperature ( - 800 K) has a closer match than room-temperature of the peak wavelength and profile to those of the observed 3.3 #m UIR feature. This enhances the belief that PAHs are ubiquitous and abundant throughout the ISM, and are responsible for the UIR by providing a better correlation of their general IR spectral characteristics. However, one of the criticisms of the PAH model raised by Leach7 and Donn, Allen, and Khannas is that the strong absorption peaks of PAH molecules in the near-UV are not evident in the interstellar UV extinction curve. This apparent conflict with the PAH hypothesis, which should be considered as a serious problem, 9 requires resolution if the PAH hypothesis for the UIR is to remain plausible. Since a large fraction of the PAHs in the general ISM are expected to be ionized, it is important that an investigation be made of the spectral properties of ionized PAHs. The need to obtain UV/visible spectra of cationic PAHs has impelled this research. In this paper, we report the UV/visible spectral properties of the PAH cations, produced by gamma irradiation of the neutral precursors coronene (CuHn), benzo[ghi]perylene (C22Hn), perylene (C2oHn) , benzo(e)pyrene (C2oHn), pyrene (Cij-Ii0), 1,2,5,6 dibenzanthracene (C22H14), and chrysene (CnHn) dissolved in boron oxide glass 03203" %H20) matrices. Details of our spectral measurements of the isolated PAHs in their neutral and cationic forms along with a discussion of astrophysical implications have been published elsewhere.~~ EXPERIMENTAL All spectra of PAHs studied in both the neutral and cationic states were obtained in a low melting inorganic glass--boron oxide glass at room temperature. The preparation of PAH molecules isolated in the glass matrix is accomplished by dispersing a small quantity ( - 0 . 1 5 % by mass) of PAH in boric acid crystals (125 mg) before heating the mixture to 240~ in an oven. On cooling the molten boric acid melt solidifies to a glass in which the PAH is presumably molecularly dispersed. The PAH cations were produced by irradiating our samples, including references without PAH, with gamma radiation from a Gamma-Cell 40 2839-Ci ~37Cs source at UAB. A thorough description of the sample preparation and the experimental techniques is reported elsewhereJ~ RESULTS AND DISCUSSION Figures l(a) and (b) show the electronic spectra of the neutral and irradiated 5-ringed perylene and benzo(e)pyrene, respectively. As shown in the figures, these neutral PAHs have few or no visible features but do have very strong absorption peaks in the near-UV. However, the irradiated PAH samples, presumably composed of both neutral and ionized species, exhibit a pronounced weakening of the near-UV features with increase in the dose of gamma irradiation. There is a saturation in the conversion of the neutral species to the cation as the dose is increased. The changes

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in intensity of the absorption bands with respect to increasing irradiation doses are plotted in Figures 2(a) and Co) for perylene and benzo(e)pyrene, respectively. (Notice that due to the difficult determination of the continuum in the UV, these plots do not exactly present the strength of peaks.) The effect of gamma irradiation on the growth of the cation bands and the correlated depletion of neutral species is obvious. The cation concentration increases sharply at the beginning as represented by the general behavior of the curves. The cation formation rate then reaches a steady state and after it can even decrease. When the population of PAH cations is built up at a certain level, the probability of recombination of a free electron with a singly positive ion increases substantially, resulting in a steady state in which the neutralization of a cation accompanies each creation of another cation. Moreover, the production of PAH dications may take place as the population of monocations increases by the conversion of monocations through a second ionization. The creation of PAH dications can be either direct, with effective photons of _ 20 eV, or sequential, requiring photons of > 7 eV for the first ionization and > 13 eV for the second from the readily singly ionized PAHs, as has been shown by a detailed examination of experimental data on the ionization potentials of PAHs. TM Incomplete removal of the near-UV absorption bands in a highly irradiated sample is attributed to the fact that some of the neutral PAH material is not dissolved in boron oxide glass, remaining as clumps. Ionization would be precluded for these clumps. The reduction of the strength of the near-UV absorption bands in samples of the matrix-isolated perylene and benzo(e)pyrene when ionized suggests why PAHs are not observed in the ISM at UV wavelengths. Similar results are found for the non-compact 1,2,5,6 dibenzanthracene (5 rings) and chrysene (4 rings) and for the other three compact PAHs studied as we reported previously. 1~ Although it is expected that the IR spectra of the cations differ from those of the corresponding neutral species, which might exclude PAH cations as the UIR carriers, recent IR measurements of cationic PAHs by Szczepanski and Vala~3 indicate that the UIR emission bands cannot be explained solely on the basis of neutral PAH species, but that cations must be a significant, and in most cases the dominant, component. This ensures that PAH molecules, expected to be ionized in a large fraction, are evident in the infrared in UIR emission. Theoretical calculations employing ab initio methods of the spectra of some cationic PAH species lead to a much better agreement between the calculated CH/CC vibration intensity ratios and those deduced from observations. However, the 3.3 #m CH stretching band of the matrix-isolated PAH cations such as coronene, ~4 perylene, pyrene, anthracene, and naphthalene13 is not found in laboratory studies utilizing low-temperature argon as the matrices. If the disappearance of this band is not a matrix effect of argon ice but an inherent spectroscopic property of cationic PAHs, the 3.3 #m UIR band might be more likely due to neutral PAHs. Our experiments also indicate, as suggested by Salama and Allamandola for naphthalene~5 and pyrene, x6 that a broad absorption continuum from the near-UV to visible apparently does develop when PAH species axe ionized (see Figure 3 for 1,2,5,6 dibenzanthracene and chrysene, and ref. 10 for the other five compact PAH species studied). This continuum absorption, as Salama and Allamandola have

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proposed, may provide the channel by which interstellar UV/visible radiation is efficiently converted to the discrete IR emission. Our work has demonstrated that the continuum phenomenon is general for all PAH species when ionized. The increase in strength with decrease in wavelength is likely not a result of an effect such as electron scattering. As matter as a fact, Treinin in a survey study on inorganic glasses for trapped radicals has pointed out that no spectroscopic indication of trapped electrons could be detected, implying that the electrons are supposed to attach to the solute molecules to produce the mononegative ions. 17 However, there is a chance of this continuum to be due to the matrix effect if Coulomb force perturbs between the constrained neighboring PAH cations and closely spaced suborbitals of higher electronic energy levels overlap more greatly. The progressive increase of the continuum with irradiation doses, compared with the saturation (and the eventual dropping) of the cation bands, suggests the formation of this continuum may be associated with the interaction between the neighboring, positively charged PAH molecules, which would be present even for dications formed in the matrix if there are any. If the broad continuum could be proved as an intrinsic characteristic of cationic PAHs, it will improve our understanding of the how PAH ions convert interstellar UV/visible energy to the IR emission as observed in the UIR. CONCLUDING REMARKS The work directed, toward seeking to provide an experimental foundation for resolution of the conflict between PAH-UIR hypothesis and the nature of the interstellar extinction curve, has resulted in several conclusions. The main result of our work concerns the behavior of UV/visible spectral feature intensities when comparing neutral and positively charged PAHs. Proponents of the PAH hypothesis countering the objection based upon a lack of observed nearUV absorption features have suggested that the mix of PAH molecules likely to exist in interstellar space would result in the "smearing out u of the individual UV features.18 This "fix" ignores the electronic properties of PAHs affected by ionization. PAH molecules are expected to be ionized in the general ISM and as such they would not exhibit near-UV absorption features between the bluest DIB absorption band at 4428 A and the UV extinction bump at 2175 A. Therefore a smearing out is not required. Ionized PAHs are still to be expected to make their presence known by emissions due to transitions similar to those between vibration states of neutral PAHs, producing the UIR bands. This result, thought to be true 1~ and now confirmed by others, 13 along with the lack of the band structure in the near-UV portion of the interstellar extinction curve, on the basis of our experiments, strongly argues for the interstellar PAHs being ionized. This is consistent with the idea that the ejected photoelectrons having excess kinetic energy are heating H I regions. ~9 There does appear to be a broad and intense absorption continuum for the ionized PAHs which extends from the UV to the visible or even the near IR. If this is an intrinsic property of individual PAH cations, this continuum might serve as an efficient channel for pumping the UIR. The structureless feature would also

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contribute to the visible portion of the interstellar extinction curve and blend but undeteetable. Further investigation on the apparent absorption continuum remains as an important goal. Recognizing the importance of examining other observational conflicts regarding the interstellar PAH hypothesis such as the lack of blue fluorescence as pointed out by Donn et al., 8 we have started a fluorescence study of PAHs matrixisolated in boron oxide glass matrices. We have found that when matrix-isolated pyrene is irradiated with a resulting reduction in strength of - 15 % of the 275-nm absorption band due to ionization of some of the molecules, the fluorescence bands between 360 and 440 nm weaken by greater than 99%! This preliminary result indicates that the irradiated boron oxide glass matrix serves to quench the fluorescence, suggesting that if PAHs exist in grain mantles they would not emit structured, observable visible radiation. We thank NASA grant NAGW-749. REFERENCES 1. K. Sellgren, in Dusty Objects in the Universe, edited by E. Bussoletti and A. A. Vittone (Kluwer, Dordrecht, 1990), p. 35, and the references therein. 2. W . W . Duley and D. A. Williams, Mon. Not. R. Astron. Soe. 196, 269 (1981). 3. A. L6ger and J. L. Puget, Astron. Astrophys. 137, 1_,5 (1984). 4. L . J . Allamandola, A. G. G. M. Tielens, and J. R. Barker, Astrophys. J. 290, 1..25 (1985). 5. L.J. Allamandola, A. G. G. M. Tielens, and J. R. Barker, Astrophys. J. Suppl. Ser. 71, 733 (1989). 6. G . C . Fliekinger and T. J. Wdowiak, Astrophys. J. 362, L71 (1990). 7. S. Leach, in Polycyclic Aromatic Hydrocarbons and Astrophysics, edited by A. L6ger, L. B. d'Hendecourt, and N. Boecara (Reidel, Dordrecht, 1987), p. 99. 8. B . D . Donn, J. E. Allen, and R. K. Khanna, in Interstellar Dust, edited by L. J. Allamandola and A. G. G. M. Tielens (Kluwer, Dordrecht, 1989), p. 181. 9. See, for example, A. Evans, The Dusty Universe (Ellis Horwood, N. Y., 1993), p. 192. 10. W. Lee and T. J. Wdowiak, Astrophys. J. 420, L127 (1993). 11. T. J. Wdowiak and W. Lee, J. Chem. Soc. Faraday Trans. 89, 2308 (1993). 12. A. T. Tokunaga, K. Sellgren, R. G. Smith, T. Nagata, A. Sakata, and Y. Nakata, Astrophys. J. 380, 452 (1991). 13. J. Szczepanski and M. Vala, Nature 363, 699 (1993) and the references therein. 14. L. d'Hendecourt and A. L6ger, in Proceedings of the First Symposium on the Infrared Cirrus and Diffuse Interstellar Clouds, in press. 15. F. Salama and L. J. Allamandola, Astrophys. J. 394, 301 (1992). 16. F. Salama and L. J. Allamandola, Nature 358, 42 (1992). 17, A. Treinin, in Radical Ions, edited by E. T. Kaiser and L. Kevan (Interscience Publishers, N. Y., 1968), p. 525. 18. C. Joblin, A L6ger, and P. Martin, Astrophys. J. 393, L79 (1992). 19. E. L. O. Bakes and A. G. G. M. Tielens, Astrophys. J., in press.


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DISCUSSION

SARRE - - Do we k n o w if these PAH cations fluoresce in the gas p h a s e ? WDOWIAKIt is i m p o r t a n t to determine if P A H cations in the gas p h a s e e m i t light w h e n excited w i t h ultraviolet radiation. We h a v e n o t p e r f o r m e d such an e x p e r i m e n t to date. H o w e v e r I can tell y o u the p r e l i m i n a r y results of an e x p e r i m e n t p e r f o r m e d w i t h boron oxide glass matrix isolation. We excited disolved p y r e n e at its 275 n m b a n d and observed its characteristic s t r u c t u r e d e m i s s i o n b e t w e e n 360 n m a n d 420 nm. The sample was t h e n i r r a d i a t e d w i t h g a m m a rays w i t h the result that the 275 n m absorption b a n d w a s r e d u c e d b y 15 percent. We were surprised to find that g a m m a irradiation h a d q u e n c h e d the UV excited emission to a level below one percent of w h a t h a d been m e a s u r e d previously. Obviously this m u s t be a matrix effect indicating a gas phase experiment is m a n d a t o r y , even t h o u g h more difficult to perform.

B U S S O L E T T I - Are y o u able to q u a n t i f y the irradiation yield? Is this v a l u e similar to a n y actual astrophysical situation ? W D O W I A K - O u r experiment is not intended to be an astrophysical simulation and the technique utilized was carried out because of the ease w i t h w h i c h m a n y s a m p l e s could be investigated simultaneously. Also w e d i d n o t h a v e access to a p o w e r f u l ultraviolet source such as a h y d r o g e n lamp, b u t d i d have access to one of the m a n y Cs 137 sources used in biomedical research at our university. The use of a g a m m a source makes quantification of y i e l d difficult because of the complexity of the process by which g a m m a rays absorbed in the sample result in ionization. It is not a direct one as w o u l d be w i t h a n ultraviolet source.

B R E C H I G N A C - I w i s h to c o m m e n t about the appearance of the UV contin u u m t o g e t h e r w i t h the b a n d s attributed to the cations. If this c o n t i n u u m is i n d e e d d u e to the cations it is m o s t probably d u e to h i g h e r - l y i n g electronic transitions. But in y o u r experiment, as well as in rare gas matrix experiments, there is an i m p o r t a n t b r o a d e n i n g of the bands d u e to the matrix. T h e n it is n o t h i n g b u t o b v i o u s w h e t h e r these electronic t r a n s i t i o n s w o u l d k e e p a c o n t i n u u m character in the case of gas phase cations, as e x p e c t e d for the carriers of the DIB's.

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W D O W I A K - Salama and Allamandola u p o n observing the apparent contin u u m in their experiments with naphthalene and pyrene in argon and neon matrices, w o n d e r e d if it was general for P A H cations. We can now say that it is w h e n they are isolated in matrices. Because of the potential for such a contin u u m being a channel for p u m p i n g at the UIR it is n o w important to determine if it exists for gas phase cations.

- - Y o u have shown the 3.3 ~tm band p r o d u c e d by Coronene in KBr matrix at high temperature. It fits quite well with astronomical observations. O n the other hand, gas phase coronene displays a peak shifted in wavelength. Could you comment about this discrepancy ? W D O W I A K - Our French friends (d'Hendecourt, Joblin and colleagues) have recently exhibited including at this meeting, data that show that at higher temperatures than in our gas phase experiments the wavelength of the C-H stretch shifts t o w a r d that of the UIR band. So now we have two w a y s of matching the 3.3 ~tm UIR wavelength. Also the ultraviolet excited emission experiments p e r f o r m e d on the smallest PAH's b y Barker and colleagues in Michigan also appear to exhibit a shift from our gas phase wavelength towards that of the UIR wavelength.

BOISSEL - - In our ion trap experiment, detection of p h o t o - f r a g m e n t a t i o n allows us to get spectroscopic information about truly isolated ions in the gas phase. In the case of pyrene, preliminary results show that, within the limit of the experimental errors, no absorption is present b e t w e e n 280 and 400 nm, w h i c h excludes the existence of a continuum. So, can y o u exclude that, in c o n d e n s e d phase experiment, the continuum can be d u e to the presence of electrons in the vicinity of the ion ? That could be the case either in rare gas matrix or in boron oxide environments. W D O W I A K - That is quite plausible and if so w o u l d eliminate the idea of a c o n t i n u u m a b s o r p t i o n for P A H cations as an astrophysical channel for p u m p i n g of the UIR. Again I'll repeat that our matrix isolation experiments verified the results of the Salama and Allamandola matrix isolation experiments and demonstrated that what they found was general for P A H cations in matrices. Ion trap experiments are very important and I am v e r y h a p p y that y o u are doing them.


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LEACH - 1. It w o u l d be of interest to do electron spin resonance m e a s u r e m e n t s o n the 7-irradiated samples and correlate with optical observations. 2. It is d o u b t f u l w h e t h e r PAH cations will fluorescence because of the existence of l o w - l y i n g electronic states which facilitate very efficient internal conversion to the electronic g r o u n d state. A v e r y u s e f u l t e c h n i q u e for d e t e r m i n i n g w h e t h e r ions fluorescence is the p h o t o i o n - f l u o r e s c e n c e p h o t o n coincidence m e t h o d (PIFCO) d e v e l o p e d b y Devoret, E l a n d a n d Leach (Chem. Phys. Letters, 1976) at Orsay w i t h w h i c h it is possible to m e a s u r e the q u a n t u m yield of ion fluorescence of m a s s selected ions, d o w n to 10 -5. WDOWIAKThis requires no response from m e as it is a c o m m e n t f r o m Leach rather t h a n a question.

FIELD-

M y q u e s t i o n is, w h a t is the ionization balance b e t w e e n P A H a n d

P A H + in the interstellar field or in the range of e n v i r o n m e n t s e n c o u n t e r e d in the ISM? W D O W I A K - - I do not k n o w the answer to the question, but I u n d e r s t a n d that Professor D a l g a r n o has m a d e some calculations r e g a r d i n g this matter