A laboratory investigation of the diffuse interstellar ...

A laboratory investigation of the diffuse interstellar bands and large linear molecules in dark clouds Thomas J. Wdowiak, Wei Lee, and Luther W. Beegle

Citation: AIP Conference Proceedings 312, 687 (1994); doi: 10.1063/1.46593 View online: http://dx.doi.org/10.1063/1.46593 View Table of Contents: http://aip.scitation.org/toc/apc/312/1 Published by the American Institute of Physics

A LABORATORY INVESTIGATION OF THE DIFFUSE INTERSTELLAR BANDS AND LARGE LINEAR MOLECULES IN DARK CLOUDS Thomas J. Wdowiak, Wei Lee, and Luther W. Beegle Department of Physics University of Alabama at Birmingham, Birmingham, AL 35294 ABSTRACT A laboratory synthesis via electrical discharge and rare gas cryogenic matrix isolation has produced molecules from a 0.5% methane in argon mixture, many of which exhibit absorption bands at wavelengths close to those of the diffuse interstellar absorption bands (DIBs), including the strongest and widest 4428/~ DIB. The visible spectrum also reveals absorption features due to the HCO molecule which exists in dark clouds. Those laboratory features at DIB wavelengths are stable under ultraviolet radiation emitted from a mercury vapor lamp, while the features attributed to HCO are easily bleached with visible wavelength light. The apparatus consists of a linear discharge tube excited by a high frequency (Tesla) transformer, a sapphire substrate cooled to 10 K by a closed cycle refrigerator, and spectrometers of the diode array and scanning varieties. These experiments demonstrate their potential for investigation of the questions of the general interstellar medium carriers of the DIBs and the kinds of molecules expected to have a role in the chemistry of dark molecular clouds. INTRODUCTION The diffuse interstellar bands (DIBs) were first recognized by Paul Merrill in the 1930's when it was noticed that there existed certain spectral absorption features which were common to a large variety and number of stars, thereby indicating their interstellar nature. The elucidation of the carders of these ubiquitous spectral features has been the goal of many researchers ever since. An early effort on our part resulted in production of absorption bands of species frozen in isolation in an argon matrix at 13 K, that we argued correlated in wavelength with the strongest of the DIBs. 1 Those experiments were inspired by the suggestion of A. E. Douglas that linear carbon chains were responsible for the DIBs. 2 We attempted to produce such free radical species in our original experiment using an electrical discharge of a gas mixture of 0.48% concentration of methane (CH4) in argon. Such a discharge will produce radical and ion species including CH, C2-, and C 3. As the discharge continues additional bands become apparent at wavelengths close to those of the strongest DIBs. Very importantly, a broad band at 4500 A became the first to be noticed in terms of order of appearance. This wavelength is within matrix shift limits of the bluest DIB at 4428/~. Matrix shifts occur because the excited state of the molecule interacts differently with the matrix from what the ground state does. This results in a spectral feature being at a wavelength different from what it would be for the absorber in the gas phase. We identiffed a total of 8 bands at wavelengths close to the strongest DIB wavelengths as then

9 1994 American Institute of Physics

687

688

I n v e s t i g a t i o n o f t h e Diffuse I n t e r s t e l l a r B a n d s

cataloged by Herbig. 3 These bands resisted photobleaching with a quartz envelope mercury vapor lamp. Kratschmer and colleagues (see ref. 4 and work cited therein) have reported bands at wavelengths near those of DIBs when carbon vapor is isolated in argon and then annealed by elevation of the temperature. We have discussed the possible relationships of our 4500/~ band and a similar band they found initially at 4470/~ and which appears to shift upon annealing to 4500/~5 In this report, we will discuss our return to doing the DIB related gas discharge matrix isolation experiments along with other spectral phenomena we have observed in the past and continue to study now but never reported before. The latter results may have bearing on the question of carbon species in dark molecular clouds. EXPERIMENTAL TECHNIQUE Our current experiments are similar to the earlier ones except for the use of an ion pump, different but equivalent spectroscopic instrumentation, and some changes in procedures that will be discussed. A mixture of 0.52% methane in argon (Air Products Custom Grade) is introduced at 8 PSI through a capillary into the central portion of a discharge tube, where it is excited from a metal electrode by a small Tesla coil. The metal surface of a cold finger, cooled by an APD Cryogenics, Inc., Model 202W Displex Expansion Engine refrigerator, serves as the second electrode. A mechanical and ion pump provides an initial vacuum and then the mechanical pump is closed from the system. During deposition, because of the pressure needed to sustain the discharge, the ion pump is turned off. Deposition at 13 K occurs on a sapphire disk appearing as a white, translucent frost. Spectroscopic measurements were carried out with a tungsten lamp powered at 45 W and a McPherson 1 meter spectrometer, operated by a McPherson step motor controller, with the photomultiplier signal coupled through a Pacific Photometric model 401 photometer to a 10 inch (26 cm) Technicon recorder. A Kodak heat rejection filter was inserted between the lamp and the vacuum shroud giving a smooth continuum spectrum. Bleaching experiments were carried out with a low-pressure mercury vapor lamp and the tungsten lamp. The light source was operated at a lower intensity and greater distance then our earlier experiment. The significance of this will be discussed later. DIB CANDIDATES Figure 1 shows the single beam absorption spectrum of reactive species isolated at 13 K in an argon matrix after the sample was exposed for extended time to UV radiation of a quartz-enveloped mercury vapor lamp and visible radiation from a tungsten lamp equipped with a Kodak heat rejection filter. Bands resulting from CH, C2-, C 3 and CNN are indicated. The wavelength positions of the four strongest DIBs at 4428 ,~, 5778 A, 6283 A,and 6613 A are indicated by the vertical lines intersecting the spectrum. The significance of photobleaching with visible light will be discussed later. UV photobleaching is done to remove most o f the C2- features at 4730/k and

T. J. W d o w i a k et al.

689

5778 A

6283 A 6613 A

\

I 4000

J

i

i

I

i

i

i

5000

I 6000

i

i

I 7000

Wavelength (A)

Figure 1. Single beam absorption spectrum of reactive species produced in a gas discharge of CH 4 (0.52%) and Ar (99.48%) and then matrix isolated at 13 K. Afterwards the matrix was photobleached with UV and visible light. Vertical lines indicate wavelengths of 4 strongest DIBs at 4428 ~, 5778 A, 6283/~, and 6613 A and laboratory bands near those wavelengths are also indicated. Other species are identified. See the text and spectrum A in Figure 2 for the wavelengths of photobleachable bands including C 2- which show some residual absorption in this sample. 5210 ]k and demonstrate that the bands near DIB wavelengths are stable against midUV (2000 A- 4000 ,~) radiation. This UV bleaching technique was described in our earlier paper, l It can be seen that our recent experiments produce the same features as we reported before 1 and these bands do correlate in approximate wavelength but within matrix shift limits with strong DIBs. The correlation is better for the longer wavelengths than for the blue feature (4428/~ DIB, 4500 A Lab). In addition, we have been concerned that our bands might be due to atmospheric contaminants including frozen 0 2 (the solid state equivalent of the atmospheric or telluric bands). We presented an analysis elsewhere demonstrating that the bands being due to frozen 0 2 is unlikely. 5 To reinforce our belief in this regard we have carried out an experiment that duplicates all conditions occurring in a full experiment except omitting electrical discharge. No bands of any sort appeared, including those expected of frozen 02, indicating that leakage of atmospheric 0 2 into our system in sufficient amounts to produce bands does not occur. Also, the bands that occur when the sample is discharged are reactive species, which is demonstrated by their disappearance upon warming the matrix to 45 K. This was done with the ion pump on and it was not overwhelmed by outgassing, which would have been expected if 02 had been present in sufficient quantities to produce the absorption bands. The increase in pressure from 7x10 -6 torr to 5x10 -5 torr

690

I n v e s t i g a t i o n of t h e D i f f u s e I n t e r s t e l l a r B a n d s

upon warming is what would be expected from an increase in vapor pressure of the w a r m e d frozen argon and 0.52 percent methane mixture. We conclude that the experim e n t produced bands which are not due to the absorption of frozen 0 2 . Because of the possibility that bands at wavelengths near those o f the DIBs m a y be due to reactive species produced from trace atmospheric contaminants in the Tesla coil excited discharge, we have done the experiment using argon without methane. This does not result in the formation of a blue band at wavelength of 4500/~ near that of the strong DIB at 4428 ,~, indicating that our laboratory band is a product of reactions most likely involving carbon when methane is present along with argon. This reinforces the idea that the 4500/~ laboratory band is a good candidate for being a laboratory analog for the 4428/~ DIB. The electrical discharge of argon, sans methane, does result in bands near the DIB wavelengths of 5778/~ and 6283/~. In this case such bands are likely due to reactive species derived from trace atmospheric contaminants such as N 2 and 02. A candidate for such a contaminant derived constituent is NO 3 which has been observed to have bands near the DIB wavelengths. 6'7 This result indicates that masking by bands of NO 3 derived from atmospheric contamination can be a serious problem for DIB analog generating experiments carried out with electrical discharges and ultraviolet irradiation. This includes experiments with carbon species other than methane, such as acetylene, carbon monoxide, etc. and inert gases different from argon. We are considering possible remedies including doing the experiment in an argon atmosphere as in a glove box. Also being considered as a source of contaminants is outgassing of the ion p u m p when it is turned off during the discharge because of the 0.5 torr pressure required for the discharge. We intend to install a valve between the ion p u m p and the vacuum shroud of the closed cycle refrigerator to remove this potential source. Another solution is to utilize a sealed system having no O-ring seals or valves, and where the optical substrate is cooled with gaseous helium which in turn is cooled by the closed cycle refrigerator. Care would be taken to insure the gases utilized are contaminant free. Using a closed cycle refrigerator has an advantage over use of liquid helium in that a sample can be held indefinitely. O T H E R SPECIES In addition to the spectral features described in our earlier papers 1'5 including those we have considered as candidates for being DIB laboratory analogs, the gas discharge/matrix isolation experiment produces other spectral features that are of astrophysical interest. As it turns out, their presence was not observed in our initial experiments because of operation of the tungsten light source at full power (80 W) in closer proximity to the sample than we do now. The existence in the matrix of other species became evident when we made measurements using a Cary 14 double beam spectrometer where monochromatic light is passed through the sample rather than polychromatic light, as was done in our single beam measurements. We observed a series of broad bands having a periodic altemation in strength at the wavelengths of 5028/~ (S), 5221 A (LS), 5426 A (S), 5647/~ (LS), 5903 A (S), 6164/~ (LS), 6493/~ (S) and 6825 A (LS), where S and LS indicate a sequence o f " s t r o n g " and "less

T. J. W d o w i a k et al.

691

Spectrum A

Spectrum B

I 4000

I

i

i

i

I

i

J

i

5000

i

I 6000

J

f

i

I 7000

Wavelength (A)

Figure 2. Spectrum A-single beam absorption spectrum of reactive species produced in a gas discharge of CH 4 (0.52%) and Ar (99.48%) and then matrix isolated at 13 K. Note sequence of HCO bands at 5028 A, 5221 j,, 5426 A, 5647 A, 5903/~, 6164 A, 6493 A, and 6825 A along with C 2- bands at 4730/~ and 5210/~. Spectrum B-after photobleaching with visible light there is a pronounced decrease in the strength of the HCO bands while those of C 2- remain unchanged. strong".This sequence is most likely due to the HCO molecule 8 formed because o f the presence of atmospheric oxygen as a trace contaminant. The reason for our not observing the presence of these bands during our initial experiments 1 became evident when we used a rapid scanning Rofin spectrometer capable of making ten spectra per second in the 220-850 nm range. The HCO bands which were at first very strong would weaken rapidly and disappear within several minutes. It was evident that the species responsible were being photobleached by the light of the tungsten lamp. The phenomenon was not unlike photobleaching of the C 2- bands at 4730 tk and 5210/~, except it envolved visible light rather than UV from a mercury vapor lamp as required in the C 2- case. When the matrix was raised in temperature from 10 K to 35 K or greater the bands returned in original strength and the process of visible light photobleaching followed by thermal annealing could be repeated as many times as desired. Figure 2 shows first the spectrum of a matrix populated with reactive species (A). By operating the tungsten lamp at lower power and further away from the sample we are able to obtain a single beam spectrum with minimal photobleaching and can control photobleaching when it is desired. In our initial experiments, we missed seeing the HCO bands because in operating our lamp at elevated intensity for a period of time prior to a scan to insure a stable condition, we inadvertently bleached the sample. The permanence of the C 2- bands under visible light irradiation is evident. Spectrum B o f Figure 2 shows the effect of extensive visible light photobleaching on the sample initially

692

I n v e s t i g a t i o n of t h e Diffuse I n t e r s t e l l a r B a n d s

having spectrum A. Note that the C2- bands remain, indicating the energy necessary to photobleach the other species including HCO is less than required to photoeject electrons from C2-. CONCLUSIONS AND FUTURE WORK In summary, we conclude that gas discharge/matrix isolation experiments can produce reactive species including one that is a good candidate for being a laboratory analog of the carrier of strong DIB at 4428/~.These same experiments also produce other identified carbon based molecules such as CH, C2-, C 3, CNN, and HCO. Species such as HCO are of interest because of their populating dark HI regions and having a role in the chemistry and thermal balance of those regions. Reactive species such as NO 3 formed from trace atmospheric contaminants can be a problem for experiments done for the purpose of producing DIB cartier candidates in the laboratory because they have bands that cover wavelengths regions of interest to the DIB question. Identification of the species discussed remains a high priority. Laser desorption/mass spectroscopy done on the matrices populated with these interesting reactive species should be coupled to optical spectroscopy. Also tumable laser spectroscopy is expected to have utility. This work was supported by NASA grant NAGW-749.

REFERENCES 1. T. J. Wdowiak, Astrophys. J. 241, L55 (1980). 2. A. E. Douglas, Nature 269, 130 (1977). 3. G. H. Herbig, Astrophys. J. 196, 129 (1975). 4. W. Kratschmer, Chem. Soc. Faraday Trans. 89, 2285 (1993). 5. T. J. Wdowiak, in Solid State Astrophysics, edited by E. Bussoletti and G. Strazzulla (Amsterdam, North Holland, 1991), p. 279. 6. E. J. Jones and O. R. Wulf, J. Chem. Phys. 5, 873 (1937). 7. W. B. DeMore and N. Davidson, J. Am. Chem. Soc. 81, 5869 (1959). 8. L. J. van IJzendoom, A Spectroscopic Study of Reaction Processes and Molecules in Ices of Astrophysical Interest, Ph.D. Thesis, Univ. of Leiden (The Netherlands 1985).