Abstract
High resolution IR spectra of aniline, styrene, and 1,1-diphenylethylene cations embedded in superfluid helium nanodroplets have been recorded in the 300-1700 cm -1 range using a free-electron laser as radiation source. Comparison of the spectra with available gas phase data reveals that the helium environment induces no significant matrix shift nor leads to an observable line broadening of the resonances. In addition, the IR spectra have provided new and improved vibrational transition frequencies for the cations investigated, as well as for neutral aniline and styrene. Indications have been found that the ions desolvate from the droplets after excitation by a non-evaporative process in which they are ejected from the helium droplets. The kinetic energy of the ejected ions is found to be ion specific and to depend only weakly on the excitation energy.
| Original language | English |
|---|---|
| Article number | 044305 |
| Journal | Journal of Chemical Physics |
| Volume | 136 |
| Issue number | 4 |
| DOIs | |
| Publication status | Published - 28 Jan 2012 |
| Externally published | Yes |
Funding
This work was supported by the EPFL, the Swiss National Science Foundation (NSF(CH)) through Grant No. 200020-119789, and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO). The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 226716. We would like to thank Professor W. J. Buma for stimulating discussions and the FELIX staff for their skilled technical assistance. FIG. 1. Energy level diagram indicating the resonant ionization scheme used to record IR spectra of ions (left). Timing diagram indicating temporal structure and timing of the ionization (UV) and excitation (IR) laser pulses (upper right). Schematic time-of-flight spectrum indicating the contributions of bare ions produced by the UV ionization of neutral molecules and those resulting from the IR excitation of ions in helium droplets. FIG. 2. (Upper panel) IR excitation spectrum of aniline ions in helium droplets consisting on average of 2700 atoms. (Lower panel) Expanded view of the same spectrum. The peaks denoted by an asterisk correspond to transitions of neutral aniline molecules in helium droplets, while the peak denoted by a diamond corresponds to the aniline 2 + complex. FIG. 3. IR excitation spectrum of neutral aniline molecules in helium droplets. The transition denoted by an asterisk belongs to the aniline cation. FIG. 4. IR excitation spectrum of aniline 2 + and aniline 3 + complexes in helium droplets and the calculated spectrum of aniline 2 + for the NH-N, NH-π, and head-to-tail geometries, see text for details. FIG. 5. Structures of the aniline 2 + cation calculated by DFT at the B3LYP/6-311 ++ G( df , pd ) level of theory. FIG. 6. IR excitation spectrum of the styrene cation in helium droplets (upper panel) and the theoretical spectrum based on DFT calculations at the B3LYP6-311 ++ G( df , pd ) level of theory (lower panel). FIG. 7. (Upper panel) IR excitation spectrum of 1,1-diphenylethylene cations in helium droplets. (Middle panel) The same spectrum recorded with a factor two reduced IR intensity. (Lower panel) Theoretical spectrum of the 1,1-diphenylethylene radical cation based on DFT B3LYP6-311 ++ G( df , pd ) calculations. The intensity of the strongest transition at 1086 cm −1 , denoted by an asterisk, has been reduced by a factor 4 for a better comparison with the experimental spectra. FIG. 8. (Upper panel) Time-of-flight mass spectra in the absence and presence of IR radiation resonant with the transition at 1531 cm −1 of styrene cations in helium droplets. (Lower panel) Time-of-flight spectra of styrene cations in the presence and absence of resonant IR radiation. The inset shows the difference between the two time-of-flight spectra. FIG. 9. Ion images and corresponding speed distribution of desolvated 1,1-diphenylethylene cations following excitation at 1547 and 335 cm −1 . The solid line is a fit of the data to a Maxwell-Boltzmann distribution with a temperature as indicated in the figure. FIG. 10. Translational temperature of aniline, styrene, and 1,1-diphenylethylene (DPE) cations ejected from helium droplets as function of excitation energy. The droplets consist on average of 2700 helium atoms. Table I. Comparison of the vibrational transition frequencies (cm −1 ) of the aniline radical cation. Helium droplets Gas phase a Argon tagging b 628 629/628 c 622 659 658/656 d / 660 c 652 788 785 890 889 915 913 995 996/993 d 993 1110 1107 1152 1153 1187 1188/1193 c 1327 1325/1323 d 1317 1488 1483 1520 1515 1591 1594/1593 d 1583 1645 1635 a Reference 16 . b Reference 24 . c Reference 18 . d Reference 17 . Table II. Comparison of the vibrational transition frequencies (cm −1 ) of the aniline molecule. Helium droplets Gas phase a Gas phase b Argon tagging c Argon matrix d 688 690 687 688 688–690 754 740–760 752 755 752–757 821 820–830 822 822 822 875 874 876 867 875–877 1028 1028 1027 1028–1029 1087 1077–1090 1088 1084 1085 1173 1166–1180 1175–1176 1273 1270–1287 1275 1282 1270–1287 1277 1280–1284 1503 1504.297 e 1502 1496 1503–1505 1622 1611–1628 1622 1610 1618–1624 a Reference 35 . b This work, see supplementary material for details. 37 c Reference 24 . d Reference 23 . e Reference 36 . Table III. Comparison of the observed vibrational transition frequencies of styrene radical cation and the scaled theoretical harmonic frequencies (cm −1 ) calculated using DFT at the B3LYP6-311 ++ G( df , pd ) level of theory. Also indicated are the calculated intensities (km/mol). Helium droplets Gas phase Theory Intensity 639 647 45.1 776 776 a /780 b 772 7.5 799 808 31.5 942 953 17.2 962 975 31.8 982 982 b 983 15.6 999 997 14.4 1103 1106 10.5 1193 1198 18.8 1238 1241 a 1236 19.1 1274 1283 1285 50.3 1380 1369 17.4 1416 1424 27.8 1446 1458 16.8 1473 1477 70.8 1514 1515 35.9 1532 1541 168.1 a Reference 21 . b Reference 20 . Table IV. Comparison of the observed vibrational transition frequencies (cm −1 ) of neutral styrene in helium droplets and in the gas phase. Helium droplets Gas phase a 695 695 906 905 992 990 1500 1497 a Reference 52 . Table V. Comparison of the experimental vibrational transition frequencies of the 1,1-diphenylethylene radical cation in helium droplets with the scaled harmonic frequencies (cm −1 ) calculated using DFT at the B3LYP6-311 ++ G( df , pd ) level of theory. Also indicated are the calculated transition intensities (km/mol). Helium droplets Theory Intensity 334 338 23.4 385 394 18.0 422 429 13.6 584 590 30.0 610 613 24.5 683 689 57.7 785 795 74.7 831 831 42.1 930 949 54.4 989 991 103.6 1047 1056 76.1 1181 1194 136.5 1293 1292 34.6 1372 1369 140.4 1479 1489 56.9 1551 1579 1000.1