1. NAT crystals exposed to HCl: a systematic RAIR investigation Rafael Escribano, Víctor J. Herrero, Belén Maté, Miguel Angel Moreno and Ismael K. Ortega Instituto de Estructura de la Materia, CSIC, Madrid rescribano@iem.cfmac.csic.es

2. This presentation: Systematic investigation of Nitric Acid Trihydrate (NAT)’s exposure to HCl Improve understanding of ternary systems RAIR spectroscopy, mass spectrometry Measurements on two NAT structures at a range of temperatures and two HCl pressure values

3. Experimental method NAT crystals of ~500-550 nm width  -NAT grown at 180K by co-deposition of H 2 O and HNO 3 at controlled ratio β-NAT grown by same method, at 185K and slightly lower water content. Crystals are then cooled down to the temperature of each experiment (87K to 165K) Initial spectra of  -NAT and β-NAT HCl added at selected pressure (10 -4 or 10 -5 mbar) Spectra monitored at time intervals after initial HCl addition until no further changes observed

4. Schematics

5. Experimental setup

6. RAIR spectra of  -NAT substrate

7. RAIR spectra of  -NAT substrate

8. RAIR spectrum of HCl.3(H 2 O)

9. 5 x 10 -5 mbar HCl on  -NAT– initial series

10. 5x10 -5 mbar, time progress Difference spectra: final (after HCl exposure) minus initial (  -NAT) HCl was wet!

11. 10 -4 mbar HCl series on  -NAT substrate Initial (before HCl exposure) Final (after HCl exposure)

12. 10 -4 mbar HCl series on  -NAT: differences Difference spectra: final (after HCl exposure) minus initial (  -NAT)

13. 155 K, 10 -4 mbar HCl

14. Comparison 2 pressures, HCl  -NAT 10 -4 mtorr HCl ) 10 -5 mtorr HCl

15. 10 -4 mbar HCl series on  -NAT substrate Initial (before HCl exposure) Final (after HCl exposure)

16. 10 -4 mbar HCl series on  -NAT: differences Difference spectra: final (after HCl exposure) minus initial (  -NAT)

17. Discussion (1)  -NAT High pressure HCl exposure (10 -4 mbar): T<150K: HCl adsorbed on surface, as single molecule or monohydrate  bands at 2200, 2500 cm -1 T=155K: HCl dissociates and penetrates crystal structure, forming Zundel ions H 5 O 2 + as in HCl.3(H 2 O)  bands at 1000-1500 cm -1 (broad), 1600-1700 cm -1 (strong), 2500-3000 cm -1 (broad) T=165K: similar effects. Loss of H 2 O seen as ‘negative’ bands Low pressure HCl exposure (10 -5 mbar) : Negligible effects at all temperatures

18. Discussion (2)  -NAT High pressure HCl exposure (10 -4 mbar) : General trends: same as for  -NAT but effects much less pronounced. T=165K: no interaction observed. Low pressure HCl exposure (10 -5 mbar) : no interaction either.

19. Conclusions Effects only observed for high HCl pressure (10 -4 mtorr) HCl adsorbs on surface at 87K, either as molecule or as monohydrate At 155K, HCl dissociates, penetrates NAT crystal structure and favours HCl.3(H 2 O)  -NAT more reactive than  -NAT Wet HCl: strong interactions; similar effects observed as for adsorption of HCl on water ice surfaces

20. ‘Ice group’ of the Molecular Physics Department of IEM, CSIC ‘Ice group’ of the Molecular Physics Department of IEM, CSIC www.iem.cfmac.csic.es/departamentos/fismol/hielos/hielos.htm

22. 155 K, 10 -5 mbar HCl

23. Characteristic bands:

24. Our project: * Experimental and theoretical study of atmospherically relevant systems * Low pressure cold air plasmas : study of ionic and neutral species present in lower (D and E) ionosphere layers