Chemical Physics Letters 626 (2015) 6–10 Contents lists available at ScienceDirect Chemical Physics Letters jou rn al h om epa ge: www.elsevier.com/locate/cplett Luminescence of photoactivated pristine and Cr-doped MgAl 2 O 4 spinel E.S. Artemyeva a , D.S. Barinov a , F.M. Atitar a , A.A. Murashkina a , A.V. Emeline b,∗ , N. Serpone c,∗ a Department of Photonics, Faculty of Physics, Saint-Petersburg State University, Saint-Petersburg, Russia b Laboratory “Photoactive Nanocomposite Materials”, Saint-Petersburg State University, Saint-Petersburg, Russia c PhotoGreen Laboratory, Dipartimento di Chimica, Università di Pavia, via Taramelli 12, Pavia 27100, Italy a r t i c l e i n f o Article history: Received 20 January 2015 In final form 27 February 2015 Available online 6 March 2015 a b s t r a c t This Letter reports a comparative study of the luminescence from pristine and Cr-doped MgAl 2 O 4 spinel induced by different excitation mechanisms: photoluminescence (PhL), thermoluminescence (ThL) and Photo-Induced Chemisorption Luminescence (the PhICL phenomenon) to understand the mechanism of PhICL emission. Cr-doping alters the major pathway of physical relaxation through a luminescence pathway: quenching of the luminescence associated with intrinsic defects and appearance of the lumi- nescence from Cr 3+ -states. The similarity between ThL and PhICL spectra suggest the mechanism of the PhICL phenomenon is due to electron transfer from the surface to the emission centers of luminescence; an energy transfer pathway is not precluded. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Photoactivation of metal oxides leads to various relaxation pro- cesses on the surface and in the bulk of the solids. Such relaxation processes may result in the emission of photons, (i.e. lumines- cence), which is an example of the physical pathway of system relaxation. Such physical relaxation can be either complete result- ing in restoration of the initial ground state, or else incomplete yielding a metastable excited state of the solids due to formation of electronic states within the energy band-gap in the metal oxides. The latter can manifest itself as formation of photoinduced color centers or photo-coloration [1,2]. Concomitantly, a chemical relax- ation pathway can also be realized in surface chemical processes occurring at the interface of the heterogeneous system [3]. Com- plete relaxation through the chemical pathway corresponds to the photocatalytic process, while incomplete relaxation in the hetero- geneous system results in such surface chemical modification as photostimulated adsorption. Both chemical and physical relaxation pathways are closely inter-connected to each other. The Photo-Induced Chemisorption Luminescence (PhICL [4–9]) is a remarkable and attractive phenomenon to observe in such interconnection between chemical and physical relaxation ∗ Corresponding authors. E-mail address: nick.serpone@unipv.it (N. Serpone). pathways in a single process when chemical relaxation through interaction of electron-donor molecules (H 2 , CH 4 , H 2 O, NH 3 ) with photoactivated surface active sites triggers the physical relaxation process of luminescence observed as a flash of light (the PhICL phe- nomenon). This phenomenon was originally observed by Andreev and Kotel’nikov on the photoactivated surfaces of Al 2 O 3 , BeO and MgO [4,5] as a result of dissociative adsorption of either water or dihydrogen. From the very first observation of PhICL phenomenon, which has attracted considerable attention, a question arose as to the source of the luminescence flash. Andreev and Kotel’nikov initially proposed that this luminescence was caused by relaxation of the electronically excited surface-adsorbed species such as the OH groups (Eq. (1)). O s - + RH → (OH s - ) ∗ + R → OH s - + h (1) where O s - is the photoinduced surface-active center of dissociative adsorption of hydrogen-containing electron-donor molecules RH, and the (OH s - )* and OH s - are the surface hydroxyl groups formed as a result of dissociative adsorption in their electronically excited and ground states, respectively; R denotes the radical species as products of dissociative adsorption, and h is the lumines- cence emission. However, in our study of the PhICL phenomenon occurring on the photoactivated surfaces of ZrO 2 and -Al 2 O 3 we observed that the PhICL spectra were very similar to the emission spectra of the photo- and/or thermo-luminescence of the metal oxides [7,8]. This inferred that the physical pathway http://dx.doi.org/10.1016/j.cplett.2015.02.050 0009-2614/© 2015 Elsevier B.V. All rights reserved.