Astrophysical Icy Surface Simulation under Energetic Particles and

Jul 15, 2008 - Instituto de Química and Observatório do Valongo, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil, Departamento de...
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11954

J. Phys. Chem. C 2008, 112, 11954–11961

Astrophysical Icy Surface Simulation under Energetic Particles and Radiation Field in Formic Acid D. P. P. Andrade,*,† H. M. Boechat-Roberty,‡ E. F. da Silveira,§ S. Pilling,§ P. Iza,§ R. Martinez,§ L. S. Farenzena,§ M. G. P. Homem,⊥ and M. L. M. Rocco† Instituto de Quı´mica and ObserVato´rio do Valongo, UniVersidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil, Departamento de Fı´sica, Pontifı´cia UniVersidade Cato´lica do Rio de Janeiro, Rio de Janeiro, RJ, Brazil, Sı´ncrotron, Campinas, SP, Brazil, and Departamento de Quı´mica, UniVersidade Federal de Sa˜o Carlos, Sa˜o Carlos, SP, Brazil ReceiVed: January 12, 2008; ReVised Manuscript ReceiVed: April 11, 2008

In this work, the fragmentation, survival, and chemical reactions of formic acid (HCOOH) molecules condensed at 56 K are analyzed using plasma desorption mass spectrometry (PDMS) and photon-stimulated ion desorption (PSID) in an effort to simulate the effects of energetic charged particles (e.g., cosmic rays) and radiation fields on interstellar/cometary ices. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing soft X-ray photons (535.1 eV) and energetic ions (∼65 MeV) obtained as 252Cf fission fragments. Mass spectra of positive and negative desorbed ions were obtained using a time-of-flight (TOF) spectrometer, providing information on the fragmentation pattern and abundance of the ionic species released from the icy surface. In both techniques, the major contribution to the released/desorbed ions were positively charged fragments. The production of several series of clusters, some of them with mass/charge ratios of up to 500 u/e, was observed in the PDMS spectra. Comparison between the employed techniques (photon and ion impacts) indicates that the interaction of energetic ions with formic acid ice produces a greater variety of ions than soft X-ray photon impact. This suggests that cosmic rays and high-energy solar wind particles, despite its reduced flux compared to other lower-energy particles, might play an important role in the synthesis of prebiotic molecules. 1. Introduction Formic acid (HCOOH), the simplest carboxylic acid, has been observed in several astronomical sources such as comets,1–3 protostellar ices NGC 7538:IRS9,4 condritic meteorites,5 dark molecular clouds,6 and regions associated with stellar formation.7,8 In some massive star-forming regions such as Sgr B2, Orion KL, and W51, glycine (NH2CH2COOH) and formic acid (HCOOH) have been observed. The dust emission in Sgr B2 is reproduced by a cold component (Td ≈ 13-22 K) together with a warm component (Td ≈ 24-38 K).9 The presence of widespread UV and X-ray fields could trigger the formation of photodissociation regions (PDRs). The X-ray photons are capable of traversing large column densities of gas before being absorbed. Inside dense molecular clouds, cosmic rays are the main source of ionization and dissociation, as they are capable of penetrating these regions and inducing the formation of new molecules. The complexity of these regions possibly allows for a combination of different scenarios and excitation mechanisms to coexist.9 Formic acid and glycine serve as model systems for the properties of larger and more complex amino acids or proteins, and it is necessary to understand their behavior during exposure to high-energy radiation.10 In this way, the photodissociation process and the resulting ionic fragment yields play an essential role in the evolution of interstellar chemistry. * Corresponding author. E-mail: [email protected]. † Instituto de Quı´mica, Universidade Federal do Rio de Janeiro. ‡ Observato ´ rio do Valongo, Universidade Federal do Rio de Janeiro. § Pontifı´cia Universidade Cato ´ lica do Rio de Janeiro. ⊥ Universidade Federal de Sa ˜ o Carlos.

According to the Safronov theory,11,12 most comets were formed in the Uranus-Neptune region (T > 50 K) and were dynamically expelled to form the Oort cloud (T ≈ 4 K), a large spherical structure with an estimated radius of 104-105 AU (astronomical units) surrounding the Sun. The astronomical unit is a unit of length approximately equal to the distance from the center of the Earth to the center of the Sun. The currently accepted value for the AU is nearly 150 million km. A much smaller number of comets are located in the Kuiper belt (T ≈ 40 K) at 50-500 AU.13 Some comets move toward the inner solar system as a result of perturbations caused by the planets and nearby stars. At small heliocentric distances, the cometary molecules are highly exposed to the solar radiation (e.g., photons and particles), which induces several physical and chemical reaction processes, such as ionization and molecular dissociation. As pointed out by Ehrenfreund et al.,14 the interstellar formic acid is much more abundant in the solid phase, in both interstellar and cometary ices, than in the gaseous phase (with an ice/gas ratio of 104 in the interstellar medium). This issue remains a puzzle, and more laboratory work is necessary to clarify this question. Study of the photodissociation of formic acid in the gaseous phase in the vacuum ultraviolet (VUV) region has been performed experimentally and theoretically15–17 by employing fast electrons in the energy range of 0.5-2 keV and energetic protons with energies covering 0.128-2 MeV.18 However, the results cannot explain the observed ice/gas ratio of formic acid. Boechat-Roberty et al.19 showed that HCOOH is almost completely destroyed by soft X-rays, justifying the low abundance of HCOOH in the gaseous phase. They suggested that the preferential path for the glycine formation via formic acid might go through the ice phase.

10.1021/jp800297f CCC: $40.75  2008 American Chemical Society Published on Web 07/15/2008

Astrophysical Icy Surface Simulation In an effort to simulate the effects produced by the interaction of energetic particles/cosmic rays and X-ray photons with condensed (ice-phase) interstellar and cometary organic molecules, we have performed experimental studies using the plasma desorption mass spectrometry (PDMS) and photonstimulated ion desorption (PSID) techniques on one of the most abundant and important organic interstellar/cometary molecules, formic acid. Bockele´e-Morvan et al.3 found a formic acid abundance ([HCOOH]/[H2O]) in comets of up to 0.09% at 1 AU from the Sun, which, in turn, led to the suggestion that formic acid ice found in the comets has been mixed with other ices such as H2O, CO, and CO2. No evidence has been found that pure HCOOH ice is present. However, because of the complexity of performing such experiments with mixtures, it is more appropriate to start with single molecular ices, such as pure formic acid ice. Cyriac and Pradeep20 showed that formic acid ices deposited at 10-20 K exist as a mixture of the amorphous (mainly dimer) and crystalline forms. The crystallization temperature depends on the substrate; for example, for amorphous H2O ice substrate, crystallization is completed at 40 K, and for KBr disk and crystalline ice surfaces, it occurs at 98 and 83 K, respectively. They found that dimer-to-crystalline conversion is irreversible. Thus, if formic acid is deposited at (or reaches) higher temperature, its structure stays crystalline, even if lower temperatures are reached later. Thus, the crystalline phase is thermodynamically more stable than amorphous form. On the astrophysical ice time scale, appreciable amorphous-crystalline conversion is expected to occur. It is known that, in cometary and interstellar ices, above 60 K, polymerization reactions involving H2CO, NH3, and CH3OH ice can produce compounds of high molecular weight.21 In addition, the chemistry can be actively modified by surface reactions in the region of the disk at 40-60 K using UV photons, X-rays, and cosmic rays. Dissociation and desorption processes can lead to radicals that can subsequently react to form other molecules. Moreover, molecules are present in young stellar objects (YSOs), where densities of 104-1013 molecules/ cm3 and temperatures of 10-10000 K exist over distances from a few stellar radii to many thousands of astronomical units. Thus, experimental results obtained at 56 K are relevant to the knowledge of astrophysical ices over a broad temperature range, being representative of several astrophysics environments, because the total and relative ion yields are significantly different only when a phase transition occurs. In section 2, we present briefly the experimental setup and the details of both techniques. The results in each case are reported in section 3, including an extensive discussion on the ejection of molecular clusters due to the impact of cosmic-raylike particles. In section 4, we present the astrophysics implications, and in section 5, final remarks and conclusions are presented. 2. Experimental Setup The experiments were performed at the Brazilian Synchrotron Light Laboratory (LNLS), in Campinas, Sa˜o Paulo, Brazil, using two ion desorption techniques, plasma desorption mass spectrometry (PDMS) and photon-stimulated ion desorption (PSID). The experimental apparatus (Figure 1) for PDMS was constructed by K. Wien and the LNLS staff, and details can be found elsewhere.22–25 Briefly, formic acid molecules were condensed onto a gold thin film connected to a helium cryostat. The ice temperature

J. Phys. Chem. C, Vol. 112, No. 31, 2008 11955

Figure 1. Schematic diagram of the experimental setup used for PDMS and PSID experiments, containing a time-of-flight mass spectrometer, a cryostat, R-particles, secondary-electron and fission-fragment (FF) detectors, and a quadrupole mass analyzer.

was on the order of 56 K. 252Cf fission fragments (FFs) including R-particles and highly charged heavy-atom cations with masses of 60-100 u and energies of 65 MeV were used to induce positive- and negative-ion desorption from the icy HCOOH surface. Induced ion desorption yields and chemical reaction rates are strongly enhanced at the condensed/ice phase, increasing the sensitivity of the experimental method and dramatically reducing the time of the measurements. Assuming no high-order effects, that is, assuming that two or more FFs will not impact at the same time on the same site, the employed higher projectile flux and resulting higher desorption yield make it possible to study in laboratory time what nature usually needs thousands or millions of years to achieve. The stopping power of fission fragments in ice is about 2-3 orders of magnitude higher than that produced by solar wind or cosmic rays.26 PSID experiments were performed using synchrotron radiation from the spherical grating monochromator (SGM) beam line in single-bunch mode (pulse period of 311 ns, with a width of 60 ps) at LNLS. One of the main advantages of this operating mode is the fact that the synchrotron light pulses can be used to start the experiment, an important issue for time-of-flight studies of surfaces. Moreover, using soft X-rays photons, one can excite specific atoms inside a molecule by tuning the incoming radiation as a consequence of the different chemical shifts. Therefore, this technique is element- and site-specific. The FF ion flux on the target was estimated to be on the order of 103 FF/s · cm2, which is a very low flux. Under these conditions, this process induces a local sublimation, whose damage is macroscopically imperceptible. There is no scattering that can then keep the molecules (or the ions) trapped to the surface, because the energy of the projectiles is sufficiently high. In the case of photons, a very low flux (∼1011-1012 photons/ s · cm2) was also used because of the single-bunch mode. To analyze the desorbed ions from the icy HCOOH surface, conventional time-of-flight mass spectra (TOF-MS) were ob-

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Andrade et al.

Figure 2. Scheme showing the desorption processes due to the impact of FFs (cosmic rays) on the surface (simulating interstellar dust).

Figure 3. PDMS mass spectra of formic acid ice up to m/z ) 50 u/e: (a) positive fragments, (b) negative fragments.

tained using the correlation between the start signal [given by FFs or secondary electrons in the PDMS case and by pulsed synchrotron radiation (SR) in the PSID case] and the stop signals given by the desorbed ions. The ionized fragments produced by the interaction of FFs or the photon beam were accelerated by an electric field, mass-discriminated inside a zero-electric field drift tube, and detected by two microchannel plate detectors arranged in a chevron configuration. A schematic diagram of the experimental setup used in the two techniques is shown in Figure 1. In both sets of experiments, the base pressure of the vacuum chamber was about 2 × 10-9 mbar, which increased by 2 orders of magnitude when the gas was introduced for condensation (T ≈ 56 K). Under these conditions, using data from Cyriac and Pradeep20 and from Souda,27 we found that sublimation occurs at 182 K. The maximum ice thickness was in the micrometer range. A thermocouple was used to determine the temperature of the icy sample. Because of conditions of temperature and pressure in the chamber, the formed ice was expected to condense as a mixture of amorphous and crystalline forms20 that is probably similar to the icy formic acid found in several comets and other astrophysical environments. The sample was purchased commercially from Sigma-Aldrich with purity greater than 99.5% and was further degassed through several freeze-pump-thaw cycles before the vapor was admitted into the chamber.

and velocity V) on the surface. The projectile traverses each angstrom in 10-17 s. The projectile loses part of its energy (dE/ dX ≈ -1 keV/Å) along of the pathway, modifying its velocity and charge state in a time of about t ) 10-16-10-15 s. The FF ionizes and/or excites molecules along its trajectory inside the solid. The energy transferred to the electronic system of the ice molecules forces the secondary electrons (δ) to move away from the projectile trajectory, generating a positive infratrack in the center and a negative ultratrack around, as displayed in Figure 2b. ε ≈ 1 V/Å is a typical electric field produced by the positive nuclear track just after the emission of the secondary electrons. Afterward, molecular dissociations and chemical reactions occur, causing emission of light ions such as H+ (Figure 2c). The next stage is the track relaxation, when preformed or newly formed larger chemical species are desorbed (Figure 2d) in t > 10-13 s. During the emission process, ion neutralization can also occur, as electrons of the solid target are pushed away from the region surrounding the projectile trajectory, producing an inner positive track and a negative track. A small fraction of these electrons escapes from the solid, and a net positively charged region appears around the impact site. H+ ions, for example, are strongly repelled by the positive track, whereas the H- ions formed close to the impact site are attracted toward the solid, being quenched or having their emissions delayed. Because this region is also momentarily depleted of electrons, the neutralization of H- is more likely to occur than that of H+. On the other hand, peripheral regions with respect to the impact site might be negatively charged by the low-energy secondary electrons [predominately of low energy (electronvolts), but can reach values on the order of kiloelectronvolts] from deep segments of the track and be more efficient platforms for H- emission. According to this point of view, H+ and H- ions both have relatively large desorption yields but originate from different surface regions or might be emitted at distinct times, revealing different circumstances of solid relaxation.29 Figure 3 shows the (a) positive and (b) negative mass spectra of icy formic acid in the mass/charge (m/z) range up to 50 u/e obtained by impact of energetic ions (z is typically equal to

3. Results and Discussion 3.1. Plasma Desorption Mass Spectrometry (PDMS). A schematic diagram of the desorption processes due to the impact of energetic particles (fission fragments, FFs) on the ice surface, simulating the effects of cosmic rays on interstellar/cometary ices, is presented in Figure 2. The FFs have an energy (EFF) of ∼65 MeV and a velocity (VFF) of ∼1 cm/ns (greater than the Bohr velocity). For ion beams having velocities near or above the Bohr velocity, the projectile energy and momentum are mainly transferred to the target by ion-electron interaction. Figure 2a illustrates the impact of a projectile (with charge q

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TABLE 1: Positive (PY) and Negative (NY) Ion Yields Per Impact Obtained with the PDMS Technique m/z 1 2 3 12/2 16/2 12 13 14 15 16 17 18 19 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 44 45 46 47

ion assignment (

H H2+ H3+ C2+ O2+ C( CH( CH2+ CH3+ O( OH( H2O+ H3O( C2( C2H( C2H2( C2H3+ CO+ HCO+ H2CO+ H2COH+ O2( O2H( H2O2(H2O)OH( (H2O)2( (H2O)H3O+ (H3O)2+ (H3O)2H+ CO( COOH( or HOCO( HCOOH( (HCOOH)H(

PY (×10-5) 20.3 3.41 0.702 0.008 0.037 12.4 4.63 2.99 5.13 1.74 5.27 42.0 64.3 0.248 1.13 2.50 4.70 5.51 61.1 4.78 6.37 2.56 5.42 3.58 1.86 6.60 2.24 4.83 2.08 17.5 1.91 148

NY (×10-6) 10.8

18.5

1.95 1.12

63.3 41.4

15.2 19.7

1.14 2.67

0.957 3.88 6.66 6.77

672 0.640 1.70 3.7

2.90 4.02 0.908 2.87 2.19

1.48 205 10.1

PY/NY

8.82 13.5 12.5 8.49

-

1.41 0.85 1.89

(1e). The fragment yields per ion impact are reported in Table 1, where positive and negative yields are indicated by PY and NY, respectively. Each ion yield was calculated by dividing the number of stop signals (the peak was fitted by a Gaussian function) by the number of start signals (∼106). The produced positive ions experience the Coulomb repulsion of the nuclear track, whereas the corresponding negative ions were generated by electron capture of thermalized atoms or molecules.28 The most common positive ions desorbed by this technique are H3O+, H2O+, H+, HCO+, and (HCOOH)H+, and the more intense negative ions are HOCO- (or HCOO-), H-, O-, and OH-. The positive ions generally have higher kinetic energies than the negative molecular ions.29,30 This difference between cation and anion kinetic energies is due to the positive infratrack (shown in Figure 2b), which repels the positive ions and tends to keep back the negative ions until their neutralization. The ratios between the positive and negative full widths at half-maximum (fwhm), PFWHM and NFWHM, respectively, are presented in Table 2. The H3O+ and H2O+ ions might be formed directly from water ice resulting from condensation of residual gas, but their origin is ambiguous because the following reactions might also occur31

H3+ + HCOOH f HCO+ + H2O + H2

(1)

H3+ + HCOOH f H3O+ + CO + H2

(2)

H2O + HCOOH2+ f H3O+ + HCOOH

(3)

These reactions have rates at 10 K of 4.30 × and 2.1 × 10-11, respectively.

10-9,

1.8 × 10-9,

TABLE 2: Full Widths at Half-Maximum of Some Positive (PFWHM) and Negative (NFWHM) Fragments and Their Ratios from PDMS m/z 1 12 13 16 17 19 25 26 32 33 35 36 37 45 46

ion assignment (

H C( CH( O( OH( H3O( C2H( C2H2( O2( O2H( (H2O)OH( (H2O)2( (H2O)H3O( HCOO( HCOOH(

PFWHM

NFWHM

PFWHM/NFWHM

0.01 0.09 0.10 0.19 0.14 0.13 0.21 0.19 0.27 0.23 0.30 0.25 0.25 0.31 0.32

0.008 0.05 0.05 0.09 0.09 0.11 0.13 0.11 0.18 0.15 0.14 0.17 0.23 0.24 0.28

1.3 1.9 1.6 2.0 1.4 1.1 1.6 1.7 1.5 1.5 2.1 1.5 1.1 1.3 1.2

The HCO+, HCOO(, and HOCO( (m/z ) 45 u/e) species are important in the formation of prebiotic molecules, mainly ketones, aldehydes, and amino acids. The first ionic species can be formed from HCOOH dissociation or from the dissociation of (HCOOH)b clusters [(HCOOH)b f (HCOOH)rOH- + HCO+]. This last mechanism of dissociation might explain the high yield of the (HCOOH)rOH- series and the absence of the (HCOOH)b( series.32 Moreover, a two-step reaction might occur, because formaldehyde (H2CdO) can be a dissociation product of HCOOH (HCOOH f H2CO + O+ + e-), and in this case, another reaction channel is H2CO f HCO+ + H + e-. Thus, HCO+ production will be enhanced. As suggested by Rescigno et al.,33 HOCO+ can be formed from the one-step reaction HCOOH f HOCO+ + H + e-. The dominant desorbed ion in the negative fragmentation spectrum is HOCO- or COOH-, which can be formed from two possible channels. The first is e- + HCOOH f HOCO - + H (electron capture and breaking of the CsH bond). The second channel was predicted by theory10,33 and was also observed in the gas phase: e- (1.25 eV) + HCOOH f HCOOH-# f HCOO- + H (electron capture and breaking of the OsH bond), where HCOOH-# represents the transient negative ion (TNI) formed in the initial Franck-Condon transition.10 Future experiments using deuterated formic acid species should be able to distinguish the site of cleavage in ice phase. Experiments performed by Pelc and co-workers10 showed that, in isolated formic acid molecules in the gas phase, the dominant reaction is dehydrogenation through the dissociative electron attachment (DEA) process with the maximum of the resonance located at 1.25 eV. On the other hand, studies on electron-stimulated desorption (ESD) from nanofilms of formic acid molecules showed an intense H- signal appearing within a resonant feature with a maximum at 9 eV and the complete suppression of HCOO- desorption [e- (9 eV) + HCOOH f HCOOH-# f HCOO/ + H-].34 However, Martin and co-workers35 showed that subexcitation electrons (1 eV) induce interesting chemical reactions in formic acid clusters, which did not appear in isolated formic acid spectra. Such reactions were possible only because of the interaction with clusters. For example, the dimer can react upon electron attachment along the routes e- + (HCOOH)2 f HCOO- + H + CO2 + H2 or e- + (HCOOH)2 f HCOOH- + CO2 + H2. The first of these reaction is endothermic by 119 kJ mol-1, whereas the second is expected to be exothermic. According to Martin and co-workers,35 in larger clusters, it is possible to find a variety of energetically favorable pathways containing the ionic units (MsH)-, where M ) HCOOH, and neutral compounds

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Andrade et al. TABLE 3: Positive and Negative Cluster Series Yields (Ions Per Impact) ion series (HCOOH)m H+ (HCOOH)nH 3O+ (HCOOH)pHCO 3+ (HCOOH)q(H2O)H3O+ (HCOOH)s(H2O)2H3O+ (HCOOH)tH2O + (HCOOH)V(O2)H+ (HCOOH)w(H2O)3H3O+ (HCOOH)jCOOH(HCOOH)rOH-

Figure 4. PDMS spectra showing the more intense clusters: (a) positive series, (b) negative series.

such as H2O, CO2, and CO, among others. These neutral species can then either be attached to some ionic complex or evaporated from an ionic complex in a reaction following electron attachment.35 We suggest that some fragments observed in the negative PDMS spectra such as OH-, O-, and H- might be produced by charge separation dissociation mechanisms, such as HCOOH f HCO+ + OH- or HCOOH f HCOH+ and O-. One of the main consequences of the impact of energetic particles on ice surfaces is the release of a large number of molecular clusters. Different kinds of molecular clusters are indeed observed in the positive and negative spectra. Peaks of clusters with mass/charge ratios of up to 500 u/e are seen in Figure 4. The positive-ion spectrum presents several series of clusters, but the (HCOOH)mH+ series has the highest yield. This shows that proton capture by formic acid is one of the dominant processes of surface reaction. Other series are visible in this spectrum such as (HCOOH)mH3O+. Practically all of the series show exponential decays with the enhancement of mass along the PDMS spectra. In the negative spectra, the (HCOOH)jCOOH- series exhibits the highest yield, with 1 e j e 9. The high yield of this series can justify the detection of HCOO- in astrophysics environments, and its formation was discussed above. Another observed negative series is (HCOOH)rOH-, with 4 e r e 9, which shows a systematic enhancement in the intensity as r increases (an unusual feature). Quantum-chemical calculations would be helpful to explain such behavior. As previously discussed, these ions seem to arise because of the reaction (HCOOH)b f (HCOOH)rOH- + HCO+. In Table 3, we summarize the yields corresponding to positive and negative series using 252Cf fission fragments on icy HCOOH. In this table, the series total yield (Y) was calculated as the sum of all components of the series (Y ) ∑ Yi, where Yi is the yield of the ith series member).

series term 1 1 1 1 1 1 1 1 1 1

e e e e e e e e e e

me7 ne8 pe4 qe6 se5 te3 Ve4 we4 je9 r e 10

yield (×10-3) 4.04 1.74 0.85 0.76 0.37 0.29 0.16 0.15 3.50 0.39

The weak presence of (HCOOH)(, the absence of the (HCOOH)b( series, and the high yield of the (HCOOH)H+ ion series suggest that formic acid prefers to receive a proton (and to have 24 electrons) than to receive or lose an electron (becoming a system with an odd number of electrons). If an electron is lost or captured to form either (HCOOH)b- or (HCOOH)b+, bond breakage occurs, forming stabler fragments having an even number of electrons. Similar results were previously reported by Pelc and co-workers.10 Their DEA results showed a prominent resonance peaking at 1.25 eV that decomposed into the formate anion HCOO- and hydrogen radical. However, the theoretical calculations predicted that HCOOH cannot bind an extra electron in a thermodynamically stable state. In other words, HCOOH has a negative adiabatic electron affinity.10 However, Martin and co-workers35 showed that HCOOH- can be formed as a product of electron capture by larger formic acid clusters with subsequent collisional stabilization (evaporative attachment). Observation of HCOOHmight indicate that the neutral compound has a positive electron affinity, but they could not prove that HCOOH- exists in a thermodynamically stable state, indicating only that the anion exists on the mass spectrometric time scale (tenths of microseconds).35 The positive total ion yield (PTIY) is 26.0 × 10-3, and the negative total ion yield (NTIY) is 6.05 × 10-3, as calculated from the total areas of the respective spectra, including all peaks. The ratio PTIY/NTIY ) 4.30 shows that positive ions are more easily formed than negative ions, an expected result in the sense that the yield of secondary electron emission is high. The ratio between the total yield of formic acid clusters (TYC) and the total yield of the formic acid fragments (TYF), which is TYC/ TYF ≈ 2 for the positive spectra and TYC/TYF ≈ 5 for the negative spectra, suggests that formic acid is desorbed intact, surviving the high energy transfer during the collision with the projectile. 3.2. Photon-Stimulated Ion Desorption (PSID). A schematic diagram of the desorption process due to X-ray photon interaction with the icy surface, simulating the effects of the radiation on interstellar/cometary ices, is presented in Figure 5. In Figure 5a, X-ray photons impinge on the surface, exciting/ ionizing the molecules. The photons can be absorbed, reflected, scattered by the surface, or transmitted. If an X-ray photon is absorbed (t ≈ 10-17 s), it can promote an electron to an excited state or to the continuum, creating a core hole (a hole in an internal orbital of the atom) on the molecule. As a consequence of the ionization process, photoelectrons are ejected by the surface. Secondary electrons are also formed. As shown in Figure 5b, after relaxation, X-ray fluorescence or Auger electron emission can occur, depending on the atomic number of the atom involved. Here, the atoms have a low atomic number, below 30. In this case, the probability of fluorescence vanishes,

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Figure 5. Scheme showing the desorption process due to X-ray impact on the surface (simulating interstellar dust).

and the Auger process has a probability of 1.37 As shown in Figure 5c, mainly because of Coulomb repulsion of positive holes in valence orbitals, molecular dissociation and desorption occur. In contrast to FF impact, soft X-ray photons interact directly with molecules of the surface. In this way, the fragments are desorbed mainly from the interaction region. The energy deposited into the system is enough to produce resonant corelevel transition, which means localized excitation. The final step in this process is bond scission at the site of excitation and consequently desorption. The positive- and negative-ion fragmentation patterns of the HCOOH ice obtained using the PSID technique recorded with a photon energy of 535.1 eV are presented in Figure 6. At this energy, two transitions are expected: O 1s (CsOH) f π* (CO) and O 1s (CdO) f 3s/σ* (HCO).36 The two sharp peaks observed at each extremity of the two spectra are due to scattered photons. These peaks occur every 311 ns, starting at 72 ns for this particular setup. In contrast to the other peaks, their positions in the time-of-flight spectrum do not depend on the extraction potential. Assignment of PSID spectra obtained in single-bunch mode is not an easy task, because the time window of 311 ns from the single-bunch mode is small enough even for the hydrogen ion. This means that H+ and heavier fragments are detected at different starts (or cycles) of our experiment, which might cause the overlapping of many different species. However, because of the very good reproducibility of the SR pulses and the good resolution achieved in these experiments (the fwhm values were ∼250 ps for the SR signal, ∼1.3 ns for H+, and between 5 and 20 ns for the other ionic species), it was possible to identify different structures and suggest the assignment of the TOF spectra as shown in the Figure 6. The fragmentation pattern of the positive ions is clearly more pronounced than that of the negative ions. In Table 4 are listed the ions formed by this technique and their positive and negative yields (PY and NY). The assignment of the most prominent positive ions, including H+; CH2+, CO2+, and/or (HCOOH)H2+; and HCO+ and/or COH+ are presented in the Figure 6a. Some ions detected in minor amounts were O+, OH+, C2+, H3O+, (HCOOH)H+, CH+, (H2O)2+, C3+, O2+, and H3+.

Figure 6. PSID time-of-flight mass spectra of formic acid ice recorded with soft X-ray photons (at 535.1 eV): (a) positive fragments, (b) negative fragments.

TABLE 4: Ion Yield Due to Photon Impact with Energy ) 535.1 eVa TOF (ns) m/z ion assignment PY (×10-6) NY (×10-6) PY/NY 79 115-127

16 24 94 17 3 153 1 171 34 178 44 12 18 220 19 231 46 236-238 13 36 36 246 58 8 262-266 47 272 28 59 277 198 289 14 14 48 309 29 310 9 a

O( C2+ (HCOOH)2H2 + OH+ H3+ H( H2O2+ CO2 + C+ H2O+ H3O+ HCOOH+ CH+ (H2O)2+ C3+ (HCOOH)C+ O2++ (HCOOH)H+ CO+ (HCOOH)CH+ (HCOOH)4CH2+ CH2+ CO2+ (HCOOH)H2+ HCO+ H2O2+

1.77 13.1

4.47 0.48 2.83

2.63

17.0

0.67

0.26

2.60 0.12 1.00

0.23 1.27 0.73 2.00 17.8 2.16 3.82

PY is positive yield, and NY negative yield.

The negative PSID mass spectrum shows only the H- and O- ions, as seen in Figure 6b. This behavior suggests higher desorption cross sections for positive ions than for negative ions. The PTIY and NTIY values obtained through the PSID data were about 5.40 × 10-5 and 1.96 × 10-5, respectively. In this case, the ratio PTYI/NTIY ) 2.75 shows that, again, positive

11960 J. Phys. Chem. C, Vol. 112, No. 31, 2008

Andrade et al.

TABLE 5: Full Widths at Half-Maximum of Some Positive (PFWHM) and Negative (NFWHM) Fragments and Their Ratios from PSID m/z 1 16

ion assignment (

H O(

PFWHM

NFWHM

PFWHM/NFWHM

1.43 5.70

1.11 2.98

1.29 1.91

ions are more easily formed than negative ions. However, the ion yield ratios O+/O- ) 0.67 and H+/H- ) 0.26 strongly suggest some favoritism in anion production for the oxygen and hydrogen atomic species. The PFWHM/NFWHM ratio shows the same behavior as in the PDMS case (Table 5). In Figure 7, we compare the HCOOH molecule fragmentation induced by different sources. On ice, the fragmentation source was 252Cf fission fragments with an energy of 65 MeV and X-ray photons at 535.1 eV. On gas, the fragmentation source was an electron beam at 70 eV (NIST), protons at 2 MeV,18 electrons at 1000 eV,18 and X-ray photons with an energy of 290 eV.19 In all cases, the yields are reported relative to the HCO+ (m/z ) 29) yield, because this is an intense fragment present in all impact regimes. This can corroborate the importance of this species to the origin of life. The figure rather reports existing data on the relative production of fragment ions from formic acid, as ice or gas. For instance, 1 keV electrons are more efficient than 70 eV electrons for producing CO+, whereas the opposite is true for HCOO+ production. Soft X-ray photons at 290 eV provide a high yield of COO group fragments (m/z ) 44-46) for the gas phase, but not at 535.1 eV for the ice phase. At 535.1 eV, the positive yield for HCOOH+ is low and that for the COOH+ fragment vanishes, suggesting a higher degree of dissociation promoted by energetic photons at the O 1s edge on surfaces. Protons with 2 MeV of energy and 1 keV electrons have about the same velocity and produce the same effect on emission from a gas target of ions lighter than HCO+; however, the results are quite different for heavier ions. It is important to note that the CO+ and CO2+ fragments do not appear in Figure 7 at 535.1 eV because their peaks are blended with others. 4. Astrophysics Implications Among the goals of astrochemistry research is to explain the existence of complex molecular species in astrophysics environments such as comets and interstellar, protostellar, and circumstellar regions.38,39 To fully appreciate the chemical path from

Figure 7. Comparison between positive-ion yields for formic acid molecules using fission fragments of 252Cf on ice, 2 MeV protons on gas, soft X-ray photons (535.1 eV on ice and 290 eV on gas), and electron impact (70 and 1000 eV on gas). In all cases, the yields are relative to HCO+ (m/z ) 29).

atoms and ions to prebiotic molecules, it is necessary to understand how the precursor species are formed. It is impossible to observe the physicochemical process directly in astrophysics environments, but it is possible to simulate it in the laboratory and empirically establish the reaction mechanisms and rates through detailed studies.40 These experimental data are required for astrochemical models, which we believe to contribute to the understanding of the differences between the chemical evolution of hot and cold regions in diverse astrophysics environments. The comparison between the two techniques used in this work indicates that high-energy ion impact (PDMS) on formic acid ice produces a greater variety of ions, mainly negative, than does interaction with soft X-ray photons (PSID). Our results show that the dissociation of formic acid ice (HCOOH) by cosmic rays is highly efficient in forming important negative ions, such as the formate (HCOO-). ISO-SWS (short-wavelength spectrometers onboard the Infrared Space Observatory) observations of the obscured young stellar object W 33A reveal two broad absorption features centered at 7.24 and 7.41 µm that correspond to formic acid (HCOOH) and formate ion (HCOO-), respectively.41 Gibb et al.42 also confirmed the detection of these species in W 33A and in several others sources. Other negative ions are also formed efficiently, such as OH-, HCO-, CO-, and CO2-, among others. Moreover, a high variety of negative clusters including formate and hydroxyl ions are formed, such as (HCOOH)jCOOH- and (HCOOH)rOH-. In addition, this work shows the high efficiency of cosmic rays in forming important positive ions, such as HCOO+ or HOCO+, and protoned cluster series. The high yield of these ions is an important result, because these species are important for the formation of prebiotic molecules, mainly for amino acids. This variety of ions might suggest that cosmic rays and higher-energy solar wind particles, despite the reduced flux in comparison to other lower-energy particles, play an important role in the synthesis of prebiotic molecules. Among the ions formed efficiently by cosmic rays and X-ray photons interacting with formic acid ice is HCO+. Miller43 has shown that amino acids are not formed directly in the electric discharge of a mixture containing CH4, NH3, and H2O, but are the result of solution reactions of smaller molecules produced in the discharge, in particular, reactions of hydrogen cyanide and aldehydes. Reactive H3+ ions were also found in both (PDMS and PSID) spectra of icy formic acid. This ion plays an important role in diverse fields from chemistry to astronomy such as the reactions that lead to the production of many complex molecular species observed in the interstellar medium.45 Recently, Pilling et al.44 proposed a new and alternative source of H3+ via soft X-ray photodissociation of abundant gaseous methyl-containing organic molecules (e.g., CH3OH, CH3NH2, CH3CN) inside dense molecular clouds, where energetic particles and photons are the main energy source. The ions desorbed from the surface of the ice can participate in new ion-molecule reactions in the gas phase. These newly formed molecular species can accrete on the surface again, enriching the ice composition and allowing the formation of larger molecules. The successive cycles of desorption and adsorption processes could lead to the formation of increasingly complex molecules. 5. Conclusions As part of a systematic experimental study of condensed (icephase) carboxyl acids, we have performed PDMS (plasma

Astrophysical Icy Surface Simulation desorption mass spectrometry) and PSID (photon-stimulated ion desorption) studies on formic acid using energetic ions and soft X-ray photons, respectively. Several fragments were identified, and their yields were determined. Among the more intense positive ions desorbed by the PDMS technique are H3O+, H2O+, H+, HCO+, and (HCOOH)H+, whereas the more intense negative ions are HOCO- (or COOH-), H-, O-, and OH-. FF impact also leads to the desorption of several positive and negative cluster series (m/z > 46), which are formed as (HCOOH)R(, where R( can be H+, H3O+, HCO3+, COOH-, or OH-, among others. The high ratio between the total yield of the formic acid clusters (TYC) and the total yield of the formic acid fragments (TYF) suggests that the formic acid is desorbed intact, surviving the impact of the high-energy ions. In this way, formic acid can be encountered free to react with others fragments, thereby forming more complex molecules. The assignment of the most prominent positive ions desorbed by the PSID technique was also presented. The negative PSID mass spectrum shows only H- and O- ions. In both techniques, the fragmentation and cluster patterns of the positive ions are clearly more pronounced than those of the negative ions. This behavior suggests higher desorption cross sections for positive ions than for negative ions. However, the ratios O+/O- < 1 and H+/H- < 1 obtained by the PSID technique strongly suggest some favoritism in anion production due to X-ray photon impact on condensed formic acid, regarding oxygen and hydrogen atomic species. All ions formed by photon interaction are also formed by the interaction with 252Cf fission fragments, but the opposite is not true. Species such as H3+, CO+, and HCO+ are important ions for the formation of more complex molecules. The analysis of pure formic acid ice was a necessary step for future treatment of ice mixtures. In conclusion, the present work help to explain, qualitatively and quantitatively (determining their ion yields), how some precursor species are formed by the impact of cosmic rays and X-ray photons on formic acid ice, pointing out the important role of ionic species in the evolution of molecular abundance and complexity of several astrophysics environments. Acknowledgment. The authors thank the staff of the Brazilian Synchrotron Facility (LNLS), mainly Dr. Arnaldo Naves de Brito, for their valuable help during the experiments. This work was supported by LNLS, CNPq, FAPERJ, CAPES, and CLAF. References and Notes (1) Crovisier, J.; Bockele´e-Morvan, D. Space Sci. ReV. 1999, 90, 19. (2) Crovisier, J.; Bockele´e-Morvan, D.; Colom, P.; Biver, N.; Despois, D.; Lis, D. C. Astron. Astrophys. 2004, 418, 1141. (3) Bockele´e-Morvan, D.; Lis, D. C.; Wink, J. E.; Despois, D.; Crovisier, J. Astron. Astrophys 2000, 353, 1101. (4) Schutte, W. A.; Boogert, A. C. A.; Tielens, A. G. G. M.; Whittet, D. C. B.; Gerakines, P. A.; Chiar, J. E.; Ehrenfreund, P.; Greenberg, J. M.; van Dishoeck, E. F.; de Graauw, T. Astron. Astrophys. 1999, 343, 966. (5) Briscoe, J. F.; Moore, C. B. Metic. 1993, 28, 330B. (6) Ehrenfreund, P. E.; Charnley, S. Annu. ReV. Astron. Astrophys. 2000, 38, 427. (7) Winnewisser, G.; Churchwell, E. Sterne Weltraum 1975, 14, 288. (8) Liu, S. Y.; Girard, J. M.; Remijan, A.; Snyder, L. E. Astrophys. J. 2002, 576, 255. (9) Goicoechea, J. R.; Rodriguez-Fernandez, N. J.; Cernicharo, J. Astrophys. J. 2004, 600, 214.

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