Interstellar problems and matrix solutions

Journal of Molecular Structure, 157 (1987) 255-273 Elsevier Science Publishers B . V ., Amsterdam - Printed in The Netherlands

INTERSTELLAR PROBLEMS AND MATRIX SOLUTIONS*

LOUIS J. ALLAMANDOLA

NASA/Ames Research Center, Moffett Field, CA 94035 (U.S .A.) (Received 29 September 1986)

ABSTRACT The application of the matrix isolation technique, which is but one of the experimental techniques pioneered in George Pimentel's laboratories, to interstellar problems is described . Following a brief discussion of the interstellar medium (ISM) three areas are reviewed in which matrix experiments are particularly well-suited to contribute the information which is sorely needed to further our understanding of the ISM . The first involves the measurement of the spectroscopic properties of reactive species . The second is the determination of reaction rates and the elucidation of reaction pathways involving atoms, radicals and ions which are likely to interact on grain surfaces and in grain mantles . The third entails the determination of the spectroscopic, photochemical and photophysical properties of interstellar and cometary ice analogs . Significant, but limited, progress has been made in these three areas and a tremendous amount of work is required to fully address the variety of unique chemical and spectroscopic questions posed by the astronomical observations . INTRODUCTION

Understanding the structure, composition and evolution of the interstellar medium, the material between the stars, poses one of the most significant challenges in modern astrophysics . The gas and dust which make up the interstellar medium (ISM) not only provide the material out of which the next generation of stars, planets and comets form, but also mediate the formation process itself . Most of the mass of the ISM. is in the form of-hydrogen which is a thousand times more plentiful than the next most abundant, chemically important elements oxygen, carbon and nitrogen (Table 1) . Some of these heavier elements (metals for many astronomers!) are in the gas phase, the remainder in the dust . Within the past twenty years, substantial strides have been made in our knowledge of the chemistry and composition of the species in the gas phase and, to date, more than 60 interstellar molecules have been identified . This progress is largely due to the efforts of astronomers who have measured the rotational transitions of interstellar molecules, radicals and ions which fall in the radio and microwave regions of the spectrum and who have *Dedicated to Professor George C . Pimentel . 0022-2860/87/$03 .50

© 1987 Elsevier Science Publishers B .V.

256 TABLE 1 Cosmic elemental abundances, relative to hydrogen [1 ] Element

Abundance

He 0 Ne C N Si Mg Fe S

0.1 6 .31 x 10-4 8 .3 x 10-5 3 .98 x 10-s 1 .0 X 10-4 3 .16 x 10-5 3 .16 x 10-5 2 .51 x 10-5 1 .58 x 10 -5

deduced some of the processes responsible for their formation and destruction . Until recently, however, the situation regarding the dust, the depository of about half of the heavy elements, was quite different . General characteristics such as shape and a limited size range were derived by analyzing how visible and near ultraviolet starlight was scattered, absorbed and polarized as it passed through the tenuous intervening dust clouds [2] . This, coupled with a knowledge of relative elemental abundances, permitted astronomers to constrain the composition somewhat . However, it was not until the advent of interstellar IR spectroscopy that precise statements could be made regarding the composition and amount of material in the dust, its role in interstellar chemistry and the extent of its interaction with the gas . The chemistry in the ISM is chemistry under extreme conditions . The temperatures are low where chemical processes occur ; the gas is about 10-100 K and the dust between 10 and 20 K . In these regions the pressure is exceedingly low, ranging from about 10 -24 to 10 - " mbar . In diffuse clouds where the density of hydrogen per cubic centimeter (n H ) in both atomic and molecular forms is on the order of 50 cm-', there is an intense UV field produced by the hot stars (T = 10 000 K) in the galaxy . This field has a flux of about 10' photons cm - ' s' in the photochemically important range of 1000 to 2000 A . In the denser molecular clouds where n H exceeds about 102 CM-3 , the radiation field is attenuated by a large factor (>10') . At the low temperatures and densities of molecular clouds, neutralneutral radical reactions are generally very slow and the gas phase chemistry is thought to be initiated by the cosmic-ray ionization of molecular hydrogen which subsequently interacts with other species in the gas, ultimately producing a set of different ions . Because of the Coulomb interaction, ion- neutral reactions are much faster than neutral-neutral radical reactions and often proceed at the Langevin rate [3] . Thus, a complex family of molecules, radicals and ions is further produced largely via an ion-molecule and dissociative recombination reaction network . Simultaneously, many of these species strike the dust grains . In the dense molecular clouds, this process of

257

accretion of species from the gas produces an "icy" grain mantle . This opens up an entire set of different reaction channels and the possibility of grain growth and evolution . This is further complicated by the possibility of photochemical processing of this mantle . Thus interstellar chemistry is chemistry in the gas phase, in the solid phase and at the interface . It should also be kept in mind that the ISM is far from chemical equilibrium . This is not only due to kinetic effects at the low interstellar temperatures, but also to the low flux of ionizing agents such as UV photons and cosmic rays which are characterized by temperatures much greater than the ambient temperature of the gas and dust . Modeling of the gas phase processes has attained a high degree of sophistication, with the behavior of many (not all) of the smaller observed species being reproduced reasonably well [4, 51 . Many of the important reactions have, however, never been studied in the laboratory at low temperatures and many of the reactive intermediates postulated have never even been made . The grain processes which include the accretion, migration and reaction of individual atoms or radicals with other atoms, molecules or radicals on the surface of a solid at 10 K have also largely been unexplored in the laboratory . Consequently, the outcome of the models of grain and gas chemistry rely heavily on assumptions . The spectroscopic properties from the UV through the IR and the photochemical and photophysical properties of these accreting grain mantles are also largely unknown . Thus, from the perspective of a chemist familiar with the tremendous amount of information matrix isolation studies have provided concerning reaction dynamics in low temperature solids and the spectroscopic properties of reactive intermediates, this area of astrophysics provides tantalizing challenges and possibilities . There are, at the very least, three areas in which matrix experiments can contribute information that is sorely needed to solve astrophysical problems . As with all new fields of research, the number of applications is likely to grow . Determining the spectroscopic properties of the many reactive intermediates believed to be important in various astrophysical environments, but for which no data is available, forms one such area . This will provide constraints on the interstellar gas phase chemistry models by enabling searches to be made for these species and, if detected, to eventually permit abundance determinations . Many of these species are interesting from a chemical and spectroscopic point of view in their own right . Another area involves the measurement, at cryogenic temperatures, of the reaction rates and elucidation of the reaction pathways involving the atoms, ions and molecules which are likely to interact on grain surfaces and in grain mantles . This will provide the data needed to realistically model the processes which occur in the bulk and on the surfaces of interstellar grains and comets . The third area is to determine the spectroscopic, photochemical and photophysical properties of interstellar and cometary ice analogs . By comparing the properties directly with those observed, one can deduce the actual make-up of the interstellar material and identify the properties which most influence the medium .



258 TABLE 2 Interstellar molecules' from ref . 6 Simple hydrides, oxides, sulfides, amides and related molecules H,CNH CC H, CO OCS H 3 CNH, CS OH SiS NO HNZ CH NS CH' HNO? SiO sic sic* H,O SO H2S NH3 SO, C H,* SiH,* Nitrites, acetylene CN C=CH C=-C-CH C=-C-C=-CH C=-C-CN C==-C-CO HC=-CH* H,NCN

derivatives, and related molecules HCN H 3C-CN H 3C-C=-C-CN HC=-C-CN H,C_C=-CH H(C=-C)3--CN H 3C-(C=-C)-H H(C=-C)3 CN H(C=-C)q CN H(C=-C)5-CN*

Aldehydes, alcohols, ethers, ketones, and related molecules H 3 COH HO-CH=O H,C=O H 3C-O-CH=O H,C=S H 3 C-CH 2-OH H 3 C-O-CH 3 H,C-CH=O H 3 CSH NH 2 -CH=O H 3 C=C=O

NaOH?

H,C=CH* H3 C-CH,CN H,C=CH--CN HN=C HN=C=O HOCO' HN=C=S

HC=O' HC=S+ HC=O HOC' HOC*

a Detections since 1978 are underlined .

The remainder of this paper is divided into three parts . In the next section the astrophysical setting will be described . Then experiments will be discussed in which the "classical" matrix isolation technique can be used to provide data which is of astrophysical relevance . Suggestions for some future experiments are given . Finally the interstellar and cometary ice analog type of experiment is described . THE INTERSTELLAR ENVIRONMENT

Dark molecular clouds are vast, low density objects composed of gas and dust with dimensions measured in light years (-10" cm) . In terms of hydrogen, the most abundant species in the universe, their densities range from n H = 10 3 --10 6 hydrogen molecules per cubic centimeter . These clouds contain about half of the mass of the ISM . In addition to hydrogen, the gas consists of a very small fraction of other atoms, ions and molecules (Table 2) . The most abundant of the known interstellar molecules is CO (n co - 10'

259

THE ORION NEBULA

INTENSE ULTRAVIOLET

COLD MOLECULAR CLOUD WITH IMBEDDED, INVISIBLE INFRARED SOURCES

Fig . 1 . The spectacular great nebula in Orion is produced by several bright stars in a cavity at the edge of a large, dark, molecular cloud . The parts of the cloud one can see in this photo are illuminated by the stars in the cavity . IR sources embedded in the dark cloud show an absorption spectrum similar to that shown in Fig . 2 while the region exposed to the intense UV emits an IR spectrum similar to that shown in Fig . 4 . (Photo courtesy of Lick Observatory .)

n H ), followed by polyatomics such as H 2O, NH 3 , and CH3OH (n = 10-5 -10-6 n H ) [ 5, 6] . Less abundant, but significant nonetheless, are a wide variety of others such as CN, HCN, HCO, HCO', H 2 CO, HNCO, CH 2 CO, HCOOH, NH 2HCO and CH 3NH2 . A recent review of the conditions and chemical state of interstellar clouds can be found in [6, 7] while an older review, written specifically for the chemistry community, can be found in ref . 8 . Although the mass of the dust is only about 1% of the mass of the gas and the dust-to-gas ratio is very low (n d = 10- ' 0 n H ), it is the dust which is the agent responsible for the obscuration of background starlight and makes these objects appear as "dark clouds" (Fig . 1) . This attenuation of the interstellar radiation field protects the molecules within the cloud from undergoing nearly complete photodissociation . This is not the case in the less dense (n H = 10-10 2 cm-'), diffuse interstellar clouds which are rich in atoms, ions and diatomic species such as C + , N, O, HD, CO, CH, CH+ , CN, OH [9] .



260

Interstellar dust is less well characterized . The general properties of the dust, as deduced from the observations, are reviewed in ref . 2 . The core, or nucleus, of the dust is thought to be made up of small silicate-like or carbonrich particles produced in the high density and temperature regions in the outflows from stars . These typically have a radius on the order of 0 .05 pm and in the ISM attain an equilibrium temperature of 10-20 K . Thus, apart from H2, most of the collisions between the dust particles and molecules in the gas (T < 100 K) will result in the molecules accreting onto the dust grains forming a mantle which is simultaneously irradiated by photons of A > 912 A, the Lyman limit . Even in a dark cloud, in the course of several hundred years, enough photons with A < 2000 A are present in the cloud to thoroughly photolyze the mantles [2] . Consequently, an interstellar grain mantle should be very similar indeed to a "matrix", albeit a very old one, 10 6 -10s years old, made up of the molecules H2 O, CH 3OH, NH3 and CO which has been photolyzed with vacuum UV radiation . The time scales for accretion of all the molecules onto the dust in dense molecular clouds are very short (10 6 years) with respect to the cloud lifetimes of about 30 million years . Since molecules are observed in the gas there must be a process which can eject at least some of the molecules making up the mantle back into the gas phase even though the nominal grain temperature is of the order of 10 K, a temperature at which normal evaporation A, Nm 4

3 W33 A

6

"WING" 'XCN -

7 8 9 10

20

"OCS"

YU

H2O

(CH30H)

CO

C2O 2

(CH

Y

1

T

4000

T

3000

2000

3T 2

(H O)

~ T

SILICATE

1000

P . CM -1

Fig . 2 . IR spectrum and band assignments of the interstellar object W-33A, an IR source embedded in a dense cloud . The deep absorption at 1000 cm' (10 µm) is attributed the SiO stretch in the silicate grains . All of the other absorption bands can be assigned to molecules frozen on the grains forming the interstellar mixed molecular "matrices" . (Figure from ref . 12.)



261

mechanisms cannot play a role . One possible mechanism for grain mantle evaporation is driven by radical reactions in the mantle . UV photolysis will produce radicals which can be stored in the mantle at the low interstellar grain temperatures . Sudden warmup of these photolyzed mantles to about 30 K promotes diffusion and radical recombination . The energy liberated by the reactions is sufficient to explosively evaporate the mantle [10, 111 . This entire process must be continuous with the accretion-photolysis stage producing new molecules and radicals, and the ejection stage introducing molecules into the gas phase . Given the conditions in the gas, some of these new molecules would be difficult, if not impossible, to produce in any other way . In addition, the ejection step replenishes the raw materials needed to sustain the gas phase chemistry . Fortunately, these clouds often contain embedded, newly formed stars and can be probed spectroscopically in the IR . A dense cloud with a deeply embedded star may appear black in the visible region of the spectrum, but emit very intensely in the IR . This is because nearly all of the stellar radiation is absorbed by the dust in a thin region around the star . This dust is heated to several hundred degrees and re-radiates the energy in the IR . The passage of this IR through the cloud provides us with the opportunity to measure the IR absorption spectrum of the material lying between the source and the earth . Thus, as shown in Fig . 2, the interstellar medium not only poses several intriguing chemical problems, but also provides us with the spectroscopic means by which we can begin to solve some of them . . Analysis of the mid-IR spectrum of such objects shows that the grain mantles consist mainly of H Z O and CH 3 OH . Traces of other simple molecules such as CO and NH 3 are also present . A significant amount of material, up to 40% of the carbon and oxygen available in dense clouds, is condensed in grain mantles [12-141 . Table 3 lists the mantle composition in several molecular clouds .

TABLE 3 The composition of interstellar grain mantles, from ref . 12 Species

Objects W3-IRS 5

H 20 1 .0 CH 30H 0 .81 CO 0 .05 Carbonyl 0.05 Aldehyde 0.05 Ketones 4 CH NH3 <0.10

NGC 2024 IRS 2 1 .0 1 .75 0 .22 0 .15 0 .35 0 .65 -

MonR2 IRS 2 1 .0 3 .0 0 .20 0 .64 0 .55 1 .3 < 0 .05 -

AFGL 961

1 .0 0 .87 0 .06 0 .04 0 .13 0 .21 <0 .015 <0 .10

W33A

1 .0 0.55 0.02 -

AFGL 2136

1 .0 0.66 0.04 0 .04 < 0 .005 <0 .10 <0 .10

NGC 7538 IRS 9 1 .0 0 .65 0 .16 0 .01 <0.01 <0.10



262

(c )GAS PHASE

(d) GRAIN MANTLE

10 3

10

N CO 02

-4

O 2 m

0 2

2

H O 10

-5

U 2

D CS HD

C

2O U 10-2 ¢

2 2

H 0

HNO NH

3 2

NH CHO

2

H CS 10

-6 10

1

3

4

10 n.(cm 3

)

10

5

Fig . 3 . Calculated abundances of species in the gas phase and on the mantles as a function of cloud age (a, b) and hydrogen density (c, d), n,, is the number density of the species considered, nH or nD , the hydrogen number density . Frames a and b are taken from ref. 16, frames c and d from ref. 15. The first comprehensive interstellar chemistry models which include both gas and solid phase chemistry in an interactive way have appeared in the recent literature [15, 16] . The most important grain surface reactions involve neutral radicals rather than ions, and hydrogenation is an important process . Figure 3 presents some of the results of these calculations . Clearly the gas phase and grain mantle composition can be vastly different due to the very different processes which determine the chemistry in each phase .



263

1500

3.3 3 .4 WAVELENGTH (,m)

3.5

3 .6

5

FREQUENCY (cm -1 ) 1000

9 WAVELENGTH (p .)

11

Fig. 4 . The IR emission spectrum from the reflection nebula HD44179 . While this spectrum is similar to others showing the mid-IR emission bands at 3030, 1615, 1300 and 885 cm- ', there are some spectral differences from object to object . This set of bands has recently been attributed to UV/visible pumped IR fluorescence from polycyclic aromatic hydrocarbon-like species . Part(a) is reproduced from ref . 17 and part (b) from ref . 18 .

Another important phenomenon, but for which there is little laboratory data available for comparison, is exhibited by a different class of interstellar IR objects . In regions where a dense cloud is subjected to the intense UV radiation from a nearby star, a set of strong, broad bands at 3050, 2940, 1615, 1300, 1150 and 885 cm' are emitted from the interface region (Fig. 4) . The bright, central nebula in Orion (Fig . 1) is a well-known source of this type of emission . These bands also seem to be emitted from the diffuse interstellar medium where there is sufficient ambient UV radiation . Analysis of data from the Infrared Astronomical Satellite (IRAS) indicates that this is a far more widespread phenomenon than previously thought . Current estimates are that a considerable fraction of the radiant energy of many galaxies (including the Milky Way) is emitted in these features [19,20] . Thus, understanding the carrier and elucidating the excitation- emission mechanism is important . It has recently been suggested that polycyclic aromatic hydrocarbon-like species (PAHs) are responsible [21-231 . Reviews of the observational data and theoretical "understanding" of the problem, prior to the PAH hypothesis, are given in refs . 13 and 24 and a compendium of papers dealing with the questions of PAHs in space is ref . 25 . INERT GAS MATRIX EXPERIMENTS

Spectroscopic properties of polycyclic aromatic hydrocarbons (PAHs) and PAH clusters Within the past two years, the case has been made that polycyclic aromatic hydrocarbons (PAHs) are abundant and ubiquitous throughout the interstellar medium . This largely rests on the suggestive, but far from perfect, agreement between the spectrum of PAHs with that of the so-called "unidentified" infrared emission (UIR) band spectrum which shows features at 3030, 1615, 1300 and 885 cm' (3 .3, 6.2, 7 .7 and 11 .3 pm) . This suggestion is supported by a better match of the UIR bands with the spectra of chars and soots which are presumably made up of mixtures of PAHs cross-linked

264

in an irregular and random fashion . This emission originates in many different types of astronomical object, indicating the carrier is a common constituent of the interstellar medium . A UV radiation field incident on a dusty region seems a necessary (but not sufficient) requirement to excite the spectrum . In contradiction to the better match with soot and char spectra, flux measurements indicate that species with molecular rather than particle dimensions are responsible . Consequently the imperfect "match" between the IR spectra of PAHs with the UIR bands has been taken to imply that PAH-like species are probably present in the interstellar medium . It has been postulated that the emission originates in PAHs which are partially hydrogenated, larger than those for which IR spectra are available, ionized and in electronic excited states . The picture which seems to be emerging within the framework of the PAH hypothesis is that individual species are excited into highly vibrationally excited states by the absorption of a single UV photon and these then relax via IR fluorescence . The suggestion of a previously unrecognized component containing about 1% of the carbon in interstellar space has ramifications for other spectral regions as well and may also contribute to other poorly understood phenomena ranging from the strong extinction in the UV to the weak, diffuse, absorption bands in the visible . Since there is little information concerning the spectroscopic properties of these reactive species from the UV to the IR, a considerable amount of matrix work is called for . The following points summarize some of the important aspects of this hypothesis . These are the first interstellar organic ring molecules known . These extremely complex species are as abundant as the most abundant but much simpler interstellar polyatomic molecules known . Their complexity and abundance indicates a unique chemical history . These are probably formed at the high densities and temperatures (n H = 10$ cm-3 , T = 1000 K) in the outflows from C-rich stars as the condensation nucleii of carbon grains . They may form the "missing link" connecting the gas with the grains . Their abundance implies lifetimes comparable to the cycling time of gas and dust between stars in the ISM . Thus once their spectroscopic properties are known, observations can be used to probe back in time . Certain properties of the carbonaceous component in meteorites and interplanetary dust particles (such as deuterium enrichment and vibrational spectra) are consistent with an origin in interstellar PAHs . This component may then be largely unmodified interstellar material . The PAH hypothesis is gaining wide acceptance . It is also thought that perhaps as much as half of the carbon in the interstellar medium is in carbon dust particles . This carbon may be characterized as amorphous rather than crystalline [261 . The Raman and IR spectra of amorphous carbon (soots or chars) resembles the interstellar IR emission band profile! [23] . Our view is that the interstellar amorphous carbon is largely made up of a mixture of randomly oriented, cross-linked PAHs . Summing up, we believe there is evi-

265

dence for individual PAH molecules, clusters of PAHs and amorphous carbon particles in the spectra of these various emission objects. In many regions of the interstellar medium PAHs are expected to be ionized . Furthermore those containing less than about 20 carbon atoms are expected to be dehydrogenated . In view of the very large variety of reactive PAHs conceivable and the exceedingly low vapor pressure of the parent species, this is an area in which matrix studies may be the only way to provide experimental data of astrophysical relevance . As so little is known about the spectroscopic properties of PAH ions and radicals from the UV through the IR, there is much to be done both experimentally and theoretically before the PAH hypothesis can be extended from a means to account for certain observations to the point where it can be exploited as a probe of interstellar processes and conditions . The latter point is particularly important since PAHs are such stable species and they can survive in rather harsh conditions . Andrews and co-workers [27] have successfully measured the visible absorption spectra of the naphthalene, chrysene, phenanthrene, and tetracene cations isolated in an argon matrix . The naphthalene cation spectra are sharp . Unfortunately, the larger PAH cation spectra are severely broadened . One hopes that this can be improved using a Ne matrix as has been shown to be the case for many other matrix isolated ion spectra . As the bulk of the PAH-related observations to date are in the IR, extending this work to the mid- and far-IR is important . Again, a thorough study and understanding of the general spectroscopic properties of PAHs is needed . Although a few astrophysically oriented laboratories have started to work in this area, the thrust will be very different from that motivated from a spectroscopic and chemical point of view . In the long run, a rather comprehensive spectroscopic approach is needed to make a substantial contribution to this field . Once the initial spectral matching phase is past, far more stringent questions will be asked, requiring quantitative optical properties spanning the entire range from the UV through the far IR . These data will be used not only to understand what form these species are in and how they influence the chemical processes and local environment in which they are found, but also to describe the radiative energy balance of the interstellar medium . There are certain objects for which 30-50% of the total radiative energy output is thought to originate from PAHs in one form or other . In addition to the study of individual species, the matrix isolation technique is also particularly suited to the study of PAH clusters . Using the approach pioneered and extensively exploited by Pimentel and many of his co-workers to study di-, tri-, and higher multimers of many different species, one can study how PAH clusters grow, what sort of interactions and geometries are involved, how cluster size and number of PAH units per cluster effects the spectrum, where the molecule-particle transition occurs and so on . In addition to cluster formation, it will also be important to understand how PAH clusters photochemically evolve . Again, the matrix technique is the method of choice. A study of how clusters are modified by photolysis

266

as a function of size and make-up will provide additional information which is sorely needed in astrophysics . These types of experiments mimic several of the steps by which carbon rich stars such as red giants, Mira variables and perhaps certain novae produce the enigmatic interstellar PAHs and carbon dust particles . Only when these processes are understood at the microscopic level will the many processes associated with dust formation in stellar envelopes of C-rich stars begin to be understood . Spectroscopic properties of reactive intermediates One of the initial contributions that researchers using the matrix isolation technique made to astrophysics was in providing spectroscopic constants for species too reactive or unstable to be studied by more conventional laboratory techniques . Once armed with these data, astronomers were able to calculate the radio or microwave spectrum and then search for emission from these particular species at the appropriate frequencies . Professor Weltner and his colleagues have made significant contributions to this field . For example, the discovery and reliable identification of C 2H in the Orion nebula [28], and C3N and C4H in the atmosphere of a carbon rich star [29, 30] was made possible by the ESR studies of these particular species in inert gas matrices [31, 321 . This approach has also been used to study the growth of SiO clusters, the precursors of the interstellar silicates [33] . Inspection of the list of hundreds of intermediate species postulated to play a role in interstellar gas phase chemistry shows that there are more than one can hope to characterize [34, 35] . The list can be narrowed down considerably since, in the various schemes proposed, some of these species are much more important than others . In general, the properties of these species are not known . Since many of the species on this much shorter list are very interesting from a structural, spectroscopic and chemical point of view, this is an ideal source of candidates for matrix isolation studies . Data such as the absorption spectrum (including band strengths) from the UV through the far-IR are sorely needed . These data are needed to determine the radiation field in a cloud and the lifetimes of the species in a UV rich region . In favorable cases, it may also be possible to determine molecular structure sufficiently well to deduce pure rotational frequencies and transition moments, providing a basis to search for these transitions in molecular clouds . Caution is warranted however, as there is a lot of activity in this area and some of the reaction schemes and rates are continuously subject to revision . Thus, if one's principle motivation is to provide data for intermediates which are important in interstellar chemistry, in addition to scrutinizing the lists of species and relative abundances, one would be well advised to contact a researcher active in this field before choosing a particular species . The majority of this community is acutely aware of the limitations of the models posed by the uncertainties in the physical properties of the intermediates and are very appreciative of new laboratory results .



267

Solid state reactions at cryogenic temperatures The third area in which "classic matrix techniques" can be exploited to produce astrophysically relevant information is in the determination of the feasibility and rates of reactions involving atoms, radicals and molecules at cryogenic temperatures . These data are crucial if one is to model dense interstellar cloud chemistry as they will reveal which grain surface reactions are important . In a dense cloud, atoms, radicals and molecules strike the grains and accrete on the surface . The grain temperature is thought to be about 10-15 K . At this temperature, the heavier species cannot diffuse . The lighter ones may be able to diffuse over a few sites on the surface while H atoms, which are far more abundant, can scan the entire surface by tunneling from site to site . The following list of surface reactions thought to be important is meant to be illustrative rather than exhaustive .

• + CO-* HCO • +O-*HO • + OH -> H2O • +N-> HN • + NH -> H 2N • +C-*CH O+O-*0 2

• +C->CO Others such as

• + CO -+ CO2 H2 CO + H -~ H 3 CO H3 CO + H -~ H 3COH have been postulated, but are less certain . Many other grain surface reactions have been suggested [15, 16, 36, 37a, b] . These questions lend themselves to a series of matrix studies . The types of matrix experiments which are expected to be most informative are interrupted warm-up investigations in which absorption band strengths attributed to individual species (both reactants and products if possible) are monitored . In some cases chemiluminescence from excited products which are formed during warm-up, or laser-induced fluorescence from reactants consumed or products formed, can be followed as a function of temperature . These types of studies will show whether or not activation barriers are important for specific reactions on grain surfaces .

268 INTERSTELLAR AND COMETARY ICE ANALOG EXPERIMENTS Another means by which matrix isolation techniques can be used to help solve problems in astrophysics is by eliminating the inert gas and increasing the complexity of the starting mixture . In this way, one can simulate the molecular mantles which accrete on the small (0 .05 µm), cold (10-15 K) dust particles . This is the approach that we pioneered in the Laboratory Astrophysics Group at Leiden University [381 . Laboratory simulation The major differences with the classic matrix isolation technique lie not in the techniques themselves, but in sample preparation [381 . The nature of the astrophysical problem requires the thorough photolysis of mixtures predominantly made up of the most abundant, condensible molecules known in the interstellar medium . This criterion requires the deposition of thin (0 .1 pm thick) "matrices" made up of the simple molecules CO, H 2O, NH 3 and CH3OH . The specific ratio chosen depends on the situation one is trying to duplicate . While there are several other areas such as in studies of cometary material and the ices in planetary rings in which these types of experiments will also be particularly relevant, this discussion will be restricted to the interstellar ice problem . The chemical and spectroscopic problems posed by such a complex chemical system would have been formidable, had it not been for the enormous effort expended by the many who have made matrix isolation the rich and exciting field it is today . This work provides the foundation for the astrophysical experiments . (e .g., see early reviews [39-41] ) . Molecule and radical formation The accretion phase in grain mantle evolution can be simulated by preparing a mixed molecular "matrix" . The presence of mixed molecular ices in space was demonstrated by comparing this type of laboratory spectrum with interstellar spectra [42] . These experiments have also been invaluable in unraveling the composition and chemical history of the dust in different environments . For example, based on the early work of Van Thiel, Becker and Pimentel [43], and Zimmermann and Pimentel [44] on the absorption spectra of various complexes of H 2O . Hagen et al . [45] carried out an extensive study of H 2 O in astrophysically relevant mixed molecular ices . This data has been used in numerous subsequent studies of many different objects to determine the temperature and thermal history of the dust in individual dense clouds, how much water is in the grains and, in a few cases, place strong constraints on the size and shape of the dust . Similar studies have now been carried out for several of the other interstellar absorption features [12, 141 . The photolysis processes can be simulated by irradiating the sample either simultaneously with, or subsequent to, deposition . The evolution of the



269

X(IM)

3

5

5

7 8 9 10 12

0.0

01

00

u01 z a m 0 N Q 00

m

i i I i i i I I

4000

3500

I l l I l l l l I l l l l I l l 2500 2000 1500 1000 500 V(cm)

I I I I

3000

I I

j l

Fig . 5 . The IR absorption spectrum of the mixed molecular ice H,O : CO : CH, : NH 3 (6 :2 :1 :1) as a function of photolysis with a microwave powered hydrogen discharge lamp ; (a) no irradiation, (b) 4 h irradiation and (c) 24 h irradiation . Comparison with Fig . 2 shows that the interstellar ices have a prominent band near 1470 cm' while the initial laboratory mixture does not . Photolysis produces a weak band near this position, presumably due to the production of -CH, and -CH 3 containing molecules . The interstellar 1470 cm - ' band is assigned largely to these species . Atmospheric CO, obscures the 2300-2400 cm - ' and 800-500 cm' regions in Fig . 2 . (This figure is reproduced from ref. 46.)

sample can be followed by recording the IR spectrum during the course of the experiment as shown in Fig . 5 . Spectroscopic analyses of the ices irradiated in the laboratory have relied heavily on the extensive CO matrix studies carried out by Jacox and Milligan ([47-49] and references cited therein) . This is because CO is frozen in the mantles in many interstellar clouds [50, 51] and it can readily react with many of the intermediates produced by photolysis, forming a wide variety of complex species as illustrated in Fig . 6 . The direct comparison of laboratory spectra of irradiated ices with observations in the 2100-1000 cm-' region shows that some form of energetic processing occurs in the interstellar grain mantles [12] . When an irradiated sample is allowed to warm up to room temperature under vacuum, not surprisingly, a non-volatile residue remains on the substrate . This type of material probably coats the grains in the diffuse UV rich interstellar medium and appears to contain 30-40% of the carbon .



270

*(2) FORMAMIDE

FORMALDEHYDE

HO \ ?12) CARBAMIC ACID

C-O

2

H N/

.CH3

3

H C ?(2) ACETAMIDE

FORMIC ACID *(2) C O

.O;

2

H N

,/ ~

\ C=0 ' H N/

ACETALDEHYDE *(2)

2

2

2

H N \ ? UREA

FORMAMIDE *(2)

2

H N /

C=_O

*(2) ACETALDEHYDE

H/

FORMIC ACID *12) \OH HO

?(2) ACETIC ACID

\ /

H 3C

WATER * C=O

-OH ',,,

3

/

C=0

H C H3C\ 7 ACETONE

CH B

ACETYL RADICAL

C=O

3

H C NH

CARBON DIOXIDE?

ACETIC ACID ?(2)

2

2 \

H N ?(2) ACETAMIDE

C=O

CARBAMIC ACID ?

H3C / *KNOWN IN THE INTERSTELLAR MEDIUM ?-NOT VET KNOWN IN THE INTERSTELLAR MEDIUM

Fig. 6 . Possible reaction pathways involving CO in an interstellar grain mantle . This illustrates how the intermediates H, OH, CH, and NH 2 , which are readily produced upon the photolysis of the mixed molecular icy mantles in space, can add to CO resulting in a very rich chemical mixture . To test the reaction-induced molecule ejection mechanism, the effects of photolysis on the pressure-versus-temperature behavior of these samples has been investigated . It has been found that some irradiated ices do indeed show small, but detectable, reaction-induced pressure bursts at temperatures as low as 15 K . At approximately 27 K virtually all photolyzed mixtures studied exhibit a dramatic burst in pressure which is accompanied by an intense flash of light . Depending upon the initial composition, this burst can eject between 10-100% of the sample [11] . Reactions involving the radical HCO are largely responsible for this behavior [52, 53] . There is interesting photophysics involved and a lot of work remains to be done to unravel the microscopic processes . This may be particularly relevant in describing the behavior of new comets which eject unexpectedly large amounts of gas and dust at great distances from the sun . The potential role of trapped radicals in various astrophysical processes was recognized quite early . In 1960 Donn pointed out that further work on reactions of free radicals in or on solids at low temperatures is extremely important for many astronomical problems [54] . Fortunately, thanks to the matrix isolation techniques pioneered in George Pimentel's laboratories at Berkeley, these studies are now possible .

271 Summing up this section, the correspondence between the interstellar IR absorption bands with features in the laboratory spectra of mixed molecular ices provides strong evidence for the presence of complex molecular ices in the interstellar medium . Specific details in the interstellar spectra such as band profile and width, absorption depth, and correlation between the features lend further support to this conclusion and provide insight into the nature and evolutionary state of these grain mantles . The experiments needed to further this field require a familiarity with developments in astrophysics . For example, the IR spectrometers now coming into use are capable of about 1 cm -' resolution . This is in sharp contrast with the 30 cm- ' resolution spectra published to date . Thus in the very near future data will become available which will warrant higher resolution analysis than that which has been needed in the past . Thus, as pointed out previously, consultation with people active in this area is advisable before embarking on an experimental program . CONCLUSION This paper has concentrated on the application of the matrix isolation technique to astrophysical problems . The purpose is to demonstrate that the matrix technique, which is but one of the many experimental techniques pioneered in George Pimentel's laboratories, provides a powerful tool which can be used to unravel the composition of the interstellar dust and gas and to delineate the processes which occur in the dust . To date significant, but limited, progress has been made using this approach and an enormous amount of work is required to fully address the variety of questions posed by the astrophysical observations . The current situation in which there is spectroscopic data of high quality which lies waiting for adequate interpretation will worsen substantially during the next ten years with the launch of the European "Infrared Space Observatory" (ISO) and the United States' "Space Infrared Telescope Facility" (SIRTF) and "Hubble Space Telescope" (HST) . Not only will good quality spectra become available which are unencumbered by the Earth's atmosphere, but also the sensitivity of these instruments will be such that spectra of faint objects, and objects showing only weak absorption features, will become accessible . The range of celestial objects available for scrutiny will expand, providing even further challenges to test the ingenuity and imagination of generations of astronomers, chemists, and spectroscopists . That the matrix isolation technique will play a central role in all of this is certain . George did not miss this either . On page 932 of his 1960 article with George Ewing and Warren Thompson [55] in which the first positive IR spectrum of a matrix isolated reactive free radical, (HCO), was reported they state, " . . . H atoms could react with CO in the interstellar dust particles and in comets . . ."

272

A rather prophetic, thought provoking, remark in view of the fact that in those days the interstellar medium was believed to be devoid of molecules with the exception of the diatomics CN, CH, and CH, and the discovery of CO in the gas phase was still ten years off . Only now, 26 years later, has the astronomical community accepted that CO will freeze out on cold grains in dense clouds . ACKNOWLEDGMENT

I am gratefully in debt to Xander Tielens who kindly gave of his time to carefully read and much improve this manuscript . REFERENCES 1 2 3 4

C . W. Allen, Astrophysical Quantities, Athlone Press, London, 1976 . J. M . Greenberg, in J. McDonnell (Ed .), Cosmic Dust, Wiley, New York, 1978, p . 187 . E . Herbst and W . Klemperer, Astrophys . J., 185 (1973) 505 . W. D . Langer in R. Lucas, A. Omontand and R . Stora (Eds. ), Birth and Infancy of Stars, North Holland, Amsterdam, 1984, p . 279 . 5 E . Herbst, in D . Hollenbach and H . Thronson (Eds .), Interstellar Processes, Reidel, Dordrecht, 1987, in press . 6 W . M . Irvine, F . P . Schloerb, A . Hjalmarson and E. Herbst, in D . C. Black and M . S. Matthews (Eds .), Protostars and Planets II, University of Arizona Press, Tucson, 1985, p . 579 . 7 W . M . Irvine, P . F . Goldsmith and A . Hjalnarson, in D . Hollenbach and H. Thronson (Eds .), Interstellar Processes, Reidel, Dordrecht, 1987, in press . 8 R . H . Gammon, Chem . Eng. News, October 2 (1978) 21 . 9 J. Black and A . Dalgarno, Astrophys . J ., Suppl. Ser ., 34 (1977) 405 . 10 J . M. Greenberg, Astrophysics Space Sci ., 39 (1976) 9 . 11 L . B. d'Hendecourt, L . J . Allamandola, F . Baas and J . M . Greenberg, Astron . Astrophys ., 109 (1982) L12 . 12 A. G . G . M . Tielens, L . J. Allamandola, J . Bregman, F . C . Witteborn, D . Wooden and D . M . Rank, Astrophys . J ., (1987) in press . 13 L . J. Allamandola, in M . F. Kessler and J . P. Phillips (Eds.), Galactic and Extragalactic Infrared Spectroscopy, Reidel, Dordrecht, 1984, p . 5 . 14 A. G . G . M . Tielens, L . J. Allamandola, J . Bregman, J . Goebel, L . B. d'Hendecourt and F. C . Witteborn, Astrophys . J., 287 (1984) 697 . 15 A. G . G . M . Tielens and W . Hagen, Astron. Astrophys ., 114 (1982) 245 . 16 L . B. d'Hendecourt, L . J. Allamandola and J . M. Greenberg, Astron . Astrophys ., 152 (1985) 130 . 17 T . R. Geballe, J . H. Lacy, S. E . Persson, P . J. McGregor and B . T. Soifer, Astrophys . J ., 292 (1985) 500 . 18 M . Cohen, L . J. Allamandola, A . G . G . M. Tielens, J . Bregman, J . P . Simpson, F. C . Witteborn, D . Wooden and D . Rank, Astrophys . J., 302 (1986) 737 . 19 P. F . Roche, in A . Leger and L . B. d'Hendecourt (Eds .), Polycyclic Aromatic Hydrocarbons and Astrophysics, Reidel, Dordrecht, 1986, in press . 20 J . L. Puget, in A . Leger and L . B. d'Hendecourt (Eds .), Polycyclic Aromatic Hydrocarbons and Astrophysics, Reidel, Dordrecht, 1986, in press . 21 W . W . Duley and D . A. Williams, Mon . Not . R. Astron . Soc ., 196 (1981) 269 . 22 A. Leger and J . L. Puget, Astron . Astrophys ., 137 (1984) L5 . 23 L . J. Allamandola, A . G . G . M . Tielens and J . R . Barker, Astrophys . J. Lett ., 290 (1985) L25 .

273 24 S . P. Willner, in M . F . Kessler and J . P . Phillips (Eds .), Galactic and Extragalactic Infrared Spectroscopy, Reidel, Dordrecht, 1984, p . 37 . 25 A . Leger and L . B . d'Hendecourt (Eds .), Polycyclic Aromatic Hydrocarbons and Astrophysics, Reidel, Dordrecht, 1986 . 26 A . G . G . M. Tielens and L. J. Allamandola, in Dr . Hollenbach and H . Thronson (Eds .), Interstellar Processes, Reidel, Dordrecht, 1987, in press . 27 L. Andrews, R . S . Friedman and B . S. Kelsall, J . Phys. Chem., 89 (1985) 4016 . 28 K . Tucker, M . Kutner and P . Thaddeus, Astrophys . J . Lett., 193 (1974) L115 . 29 M . Guelin and P . Thaddeus, Astrophys . J. Lett ., 212 (1977) L81 . 30 M . Guelin, S . Green and P . Thaddeus, Astrophys . J. Lett ., 224 (1978) L27 . 31 W. Graham, K . Dismuke and W . Weltner, Jr ., J. Chem . Phys ., 60 (1975) 3817 . 32 K . Dismuke, W . Graham and W . Weltner, Jr ., J. Mol. Spectrosc ., 57 (1975) 127 . 33 R . K . Khanna, D . D . Stranz and B . Donn, J . Chem . Phys., 74 (1981) 2108 . 34 W. T . Huntress, Astrophys . J . Suppl ., 33 (1977) 495 . 35 V . G . Anicich and W. T. Huntress, Astrophys . J . Suppl. Ser ., volume 62, in press . 36 W. D. Watson and E . E. Salpeter, Astrophys . J., 174 (1972) 321 . 37 (a) M . Allen and G . W. Robinson, Astrophys . J., 195 (1975) 81 . (b) M . Allen and G . W . Robinson, Astrophys . J., 212 (1977) 396 . 38 W . Hagen, L . J. Allamandola and J. M . Greenberg, Astrophys . Space Sci ., 65 (1979) 215 . 39 B . Meyer, Low Temperature Spectroscopy, Elsevier, New York, 1971 . 40 H . Hallam (Ed .), Vibrational Spectroscopy of Trapped Species, Wiley, New York, 1973 . 41 G. Ozin and M . Moskovits, (Eds .), Cryogenic Chemistry, Wiley, London, 1976 . 42 W . Hagan, L . J. Allamandola and J . M . Greenberg, Astron . Astrophys ., 86 (1980) L3 . 43 M . Van Thiel, E . D . Becker and G . C. Pimentel, J . Chem . Phys ., 59 (1957) 5199 . 44 R . Zimmerman and G. C . Pimentel, Adv . Mol . Spectrosc ., 2 (1962) 726 . 45 W . Hagen, A . G. G . M . Tielens and J . M . Greenberg, Astron . Astrophys ., 117 (1983) 132 . 46 L . B . d'Hendecourt, L . J. Allamandola, R . J . Grim and J . M . Greenberg, Astron . Astrophys., 158 (1986) 119 . 47 M . E . Jacox, D . E . Milligan, N . G . Moll and W . E . Thompson, J . Chem . Phys ., 43 (1965) 3734 . 48 D . E . Milligan and M . E . Jacox, J . Chem . Phys., 43 (1965) 4487 . 49 D . E . Milligan and M . E . Jacox, J . Chem . Phys., 54 (1971) 927 . 50 J . H . Lacy, F . Baas, L . J. Allamandola, S . E. Persson, P. J . McGregor, C . J. Lonsdale, T. R. Geballe and C . E . P. M. van de Bult, Astrophys . J., 276 (1984) 533 . 51 T. R. Geballe, Astron . Astrophys., 162 (1986) 248 . 52 L . J . Van Ijzendoorn, L . J. Allamandola, F . Baas and J . M . Greenberg, J . Chem . Phys., 78 (1983) 7019 . 53 L . J . Van Ijzendoorn, Ph .D . Dissertation, Leiden University, 1985 . 54 B . Donn, in A . M . Bass and H . P . Broida (Eds .), Formation and Trapping of Free Radicals, Academic Press, New York, 1960, p . 347 . 55 G. E . Ewing, W . E . Thompson and G . C. Pimentel, J. Chem . Phys ., 32 (1960) 927 .