Instantaneous collision complexes in molten alkali halides: Picosecond dynamics from low-frequency Raman data

Kirillov, SA; Pavlatou, EA; Papatheodorou, GN

dc.contributor.author	Kirillov, SA	en
dc.contributor.author	Pavlatou, EA	en
dc.contributor.author	Papatheodorou, GN	en
dc.date.accessioned	2014-03-01T01:52:06Z
dc.date.available	2014-03-01T01:52:06Z
dc.date.issued	2002	en
dc.identifier.issn	0021-9606	en
dc.identifier.uri	https://dspace.lib.ntua.gr/xmlui/handle/123456789/26562
dc.subject.classification	Physics, Atomic, Molecular & Chemical	en
dc.subject.other	VIBRATIONAL-ENERGY RELAXATION	en
dc.subject.other	LIGHT-SCATTERING	en
dc.subject.other	MONATOMIC SOLVENTS	en
dc.subject.other	CONTINUOUS POTENTIALS	en
dc.subject.other	COMPUTER-SIMULATION	en
dc.subject.other	COLLECTIVE MODES	en
dc.subject.other	ENSKOG THEORY	en
dc.subject.other	LIQUIDS	en
dc.subject.other	MELTS	en
dc.subject.other	SPECTRA	en
dc.title	Instantaneous collision complexes in molten alkali halides: Picosecond dynamics from low-frequency Raman data	en
heal.type	journalArticle	en
heal.language	English	en
heal.publicationDate	2002	en
heal.abstract	The picosecond dynamics of molten alkali halides is discussed, and the low-frequency Raman spectra of molten LiCl, CsCl, and the LiCl-CsCl eutectic are fitted to the model enabling to obtain the times of vibrational dephasing, tau(V) and vibrational frequency modulation tau(omega). In terms of the Wilmshurst criterion [J. Chem. Phys. 39, 1779 (1963)] and using the data of NMR studies and molecular dynamics simulations, a conclusion is drawn that molten alkali halides cannot contain long-lived stable complexes with lifetimes greater than 10(-8) s. The low-frequency Raman spectra of molten alkali halides and their mixtures probe the presence of instantaneous spatial configurations of MXn-n+1 type, where M+ is the alkali metal cation and X- is the halide anion existing in melts during the time intervals equal to the time of duration of collision of oppositely charged ions tau(d), which is less than 0.5 ps. This time is sufficient to a collision complex to execute several (at least one) vibrations. Vibrational dephasing and modulation processes elapse during this same time, thereby indicating the instantaneous nature of configurations in question. To discern between short-lived and long-lived complexes, we propose relations between the minimal damping time of the probe oscillator set equal to the half-period of vibration T/2, tau(V), tau(omega), and tau(d), as well as the time between collisions tau(BC). The duration of an act resulting in the vibrational phase shift (or energy transfer) must be equal to (or longer than) the half-period of vibration of the probe oscillator, tau(V)greater than or equal toT/2. The modulation time may vary from this same half-period of vibration or the time between collisions tau(BC) to very long times, tau(omega)greater than or equal toT/2, tau(omega)greater than or equal totau(BC). For short-lived complexes, the longest of two characteristic times describing the phase decay cannot exceed possible duration of collision, tau(omega)less than or equal totau(d), tau(V)less than or equal totau(d). Cs-containing configurations follow this definition and therefore should be considered instantaneous short-lived collision complexes: their tau(V)approximate toT/2similar to0.1 ps, and tau(omega)approximate totau(BC)similar to0.03 ps. Li-containing configurations appear to be relatively long-lived: their lifetimes could be associated with tau(omega)similar to0.17 ps, which is several times longer than any other shortest possible characteristic time in the system (tau(BC)similar to0.026 ps or T/2similar to0.05 ps). In light of these conclusions, an a priori assumption of autocomplex MX4n-4 anions and Mn+ cations as being structural elements of molten halides made in the so-called autocomplex model by Smirnov, Shabanov, and Khaimenov [Elektrohim. 2, 1240 (1966)] is discussed, and the autocomplexes are identified as instantaneous short-lived configurations detectable by the Raman method. (C) 2002 American Institute of Physics.	en
heal.publisher	AMER INST PHYSICS	en
heal.journalName	JOURNAL OF CHEMICAL PHYSICS	en
dc.identifier.isi	ISI:000175487300026	en
dc.identifier.volume	116	en
dc.identifier.issue	21	en
dc.identifier.spage	9341	en
dc.identifier.epage	9351	en