Chemical Reactions Induced by Ionizing and Electron-beam Irradiation in Freon/Water (Ice) Films

Transcription

1 Chemical Reactions Induced by Ionizing and Electron-beam Irradiation in Freon/Water (Ice) Films Johns Hopkins University (founded in 1876) Dr. C.C. Perry Prof. D.H. Fairborther School of Arts & Sciences Chemistry Department Rutgers University (founded in 1766) Dr. N.S. Faradzhev Prof. T.E. Madey Lab for Surface Modification Dpt of Physics and Astronomy

2 Chemical Reactions Induced by Ionizing and Electron-beam Irradiation in Freon/Water (Ice) Films 1. Introduction. Experimental setup utline 3. Results: - introductory characterization of the film - irradiation induced reaction; reaction kinetics (FTIR results) - volatile products (ESD and TPD studies) - chemical states as seen from XPS 4. Summary Pre-History Enhancement of F - and - yields in electron-induced dissociation of coadsorbed with PLAR molecules Possible explanation: - self-trapped electrons: low-e secondary electrons are trapped in clusters of molecules (like e - (H ) n ) with permanent dipole moments: e - + nh e s - (H ) n - trapped electrons tunneling to the molecule and form a vibrationally exicted intermediate state (like *- ): e s - + *- - dissociation of intermediate state molecule: *- CF + F - *- + - Coverage dependence intermolecular de-exitation effect decreases the dissociation probability: *- - increase of concentration leads to decrease of enhancement

3 Experimental setup gas flow IR chamber At RT: Torr Cold sample: Torr QMS flood gun shutter XPS chamber Torr x-ray source analyzer Side View freon LN air f low m anipulator Turbo Turbo, TSP, ion IR source nd manipulator analyzer Top View water detector ion gun manifold for gas admixture ~ Torr IR film analysis of chemical compounds MS analysis of volatile products XPS analysis of chemical states

4 Initial characterization of thick freon/water film: gas-phase spectra neat freon ~10-6 Torr gas phase C CF CF F C C CF CF H H freon/water + ~10-6 Torr background x5 ~5x10-8 Torr amu gas-phase spectra: - show REGULAR set of FRAGMENTS which correspond to molecular (for both neat freon and freon/water mixture films) and H (for freon/water mixture only) - D NT CHANGE for a long time (for instance, after night in manifold at RT) background spectra (shown one of the worst): trouble a lot of species like C, F, C, 78 amu, etc There is not any chemical interaction between freon and water in gas phase at RT

5 Initial characterization: IR spectra H 1099 cm -1 s-str 98 cm -1 C a-str cm -1 a-str 881 cm -1 FR Absorbance (R-R b ) cm -1 -H str freon/water H cm -1 bending T~90-95K neat freon Wavenumber, cm -1 NIST gas phase IR spectrum for Assignments: - are based on NIST gas phase spectra and earlier data obtained by D.H.Fairbrother s group (for water) and by other authors (for freon) - for H observed two wide bands; for two pairs of peaks or two doublets - spectra do not change for at least 1 hour There is not any chemical interaction between freon and water in condensed phase (on the surface) at LN temperature

6 Adsorption kinetics for freon on Au: derived from IR spectra Area of IR band (absorbance) 1144 & 1099 cm -1 bands of substrate at T~90-95K Dose, L It took a long time (30-60 min) to depose from gas phase at ~ Torr Straight line constant sticking coefficient

7 Electron-induced transformation of freon/water (ice) film: IR results Absorbance (R-R b ) shape changed -> H cm -1 -H str H C 345 cm -1 C str exposed +30 ev electrons 3x10 16 e/cm shape changed ->H 3 + initial cm -1 bending H Wavenumber, cm cm -1 υ, where υ is s-str 1936 cm -1 C str 1890 cm -1? C str of CF 143 cm -1 a-str 797 cm -1 υ 6 mode 881 cm -1 FR 98 cm -1 C a-str 1144 cm -1 a-str 1099 cm -1 s-str E-beam exposure leads to formation of new products in the film - Major products are C, and H 3 + ion - Very weak feature for phosgene (C ) at 1798 cm -1 - Decrease of molecular in the film 30 ev electrons + film of several hundred thickness -> may exist an erosion of the film Transmittance 1974 Chem. Phys. Lett., J.N.Pitts et al. Major product from reaction of ( 1 D) *) atoms with is *) ( 1 D) electronically excited oxygen atom Electrons induce chemical reactions in freon/water (ice) film which result in formation of, C and H 3 +

8 Transformation of freon/water (ice) film during X-ray irradiation: IR results 90 min of X-ray exposure 300W,15 kev Absorbance (R-R b ) initial Differencial spectrum: before and after irradiation C 345 cm -1 C str 139? HFC some times visible Wavenumber, cm -1 after 90 min of X-ray exposure 1936 cm -1 C str 1905 cm -1 s-str 143 cm -1 a-str Wavenumber, cm -1 During irradiation the same products are formed: C, and H 3 + ion Secondary electrons might be responsible for chemical reaction observed X-ray exposure also leads to formation of, C and H 3 + in the freon/water film

9 Reaction kinetics during electron bombardment in Water (1:5) C 4 in Water (1:4) 30 ev ~1x10 17 e/cm ~ e/cm 00 ev Area under IR band C /10 /3 C 4 C C Electron exposure, x10 14 e/cm Decomposition kinetics of freon at different energies Amount in the film 1 freon/water 30 ev neat freon 30 ev freon/water 40 ev freon/water 300 ev Reaction rate for is at least one order higher than for C 4 Decomposition rate of in H under electrons does not depend considerably on electron energy secondary electrons and low energy inelastically backscattered electrons might play important role in reactions observed Exposure, x10 14 e/cm Decomposition rates of neat freon and in H are very similar in water decomposes and reacts under electron bombardment much faster than C 4 / H

10 Electron stimulated desorption from freon/water film H C C H 30 ev 5x10 14 e/cm Too noisy curves when use low electron flux ESD yield (rough data) 1x10 16 e/cm 5x10 16 e/cm Major volatile products are: H,, C, H, C, and 1.3x10 17 e/cm amu ~1x10 17 e/cm 30 ev ESD yields (curves scaled) H C Decrease of and water signal accompanied by increase of H and H 1000 Exposure, x10 14 e/cm Electron irradiation of / H film leads to desorption of C, H and as well as parent molecules: freon and water

11 Thermal desorption from exposed freon/water film H H C CF C HF (some times) H gas phase C C CF ESD ~10 17 e/cm CF After exposure to ~ 3x10 17 e/cm : - (no by IR, but still is in the film by TPD though considerably less) - H - H - might be H (intensity HCFCL fragment less than intensity of CF fragment probably, product of thermal reaction No evidence of desorption of in molecular form (after irradiation) TPD: T ~15K after exposure to 3x10 17 e/cm H 51 amu? H 67, 69 amu? HCF Max desorption rate for freon: - neat ~ 110K - in water ~ 160K amu Yield 100 ~160K H 10 H 1 Temperature Perhaps, thermo-reactivity of products of electron induced chemical reaction in freon/water film may lead to formation H during heating In water maximum desorption rate for freon shifts to higher temperatures

12 Mg Kα 15 kv, 300W KLL Au 4s F 1s Electron induced transformations from the point of view of XPS F KLL Au 4p 3/ 1s Au C 1s s p B.E., ev Au 4f Au 5p substrate 9E e +4E x 5E e +3E x 3E e +E x E e +E x initial - instantly after 10 min under X-ray irradiation ( initial ) C(1s), (p) and F(1s) spectral envelopes show at least chemical stages - irradiation (electron and/or x-ray) leads to growth of a new peaks centered at lower binding energies: C(1s) broadens towards lower BE production of largely dehalogenated carbonaceous species (p) same behavior production of - F(1s) growth of new peak at lower BE production of F - appearance of Au peak during e-beam irradiation evidence of desorption/erosion C(1s) (p) F(1s) -6 ev -3.5 ev -.9 ev C ev -.5 ev 9E e +4E x 9E e +4E x 9E e +4E x 5E e +3E x 5E e +3E x 3E e +E x 5E e +3E x C1s 3E e +E x p E e +E x F 1s 3E e +E x E e +E x E e +E x initial initial initial B.E., ev B.E., ev B.E., ev Both x-ray and electron irradiations results in rapid formation of negative ions: F -, - E-beam exposure leads to fast desorption of the film whereas x-ray does not (as seen from separate experiment)

13 Most reliable results Interaction of low energy electrons with / H (ice) film results in rapid chemical reaction. There is not any chemical interaction between parent molecules neither in gas phase at RT nor in condensed phase (on the surface). Electron irradiation induces: - decomposition of freon molecules and production of, C, protonated ions H 3 + in the film. - formation in the film dehalogenated carbonaceous species as well as F - and - - electron stimulated desorption of parent molecules (freon and water) and C, H and Decomposition of under electron bombardment: - in ice film decomposes and reacts much faster than C 4 in H - decomposition rates of neat freon and in H are very similar - decomposition rate of in H does not depend considerably on electron energy secondary electrons might play an important role in reactions observed