C o n d e n s a t i o n o f E x c i t e d M o l e c u l e s u n d e r P h o t o - E x c i t a t i o n E x t e r n a l f i e l d s , s u c h a s m a g n e t i c f i e l d s a n d p r e s s u r e , f r e q u e n t l y i n f l u e n c e t h e o r d e r p a r a m e t e r s o f t h e p h y s i c a l s y s t e m a n d t h u s c a u s e p h a s e t r a n s i t i o n s . I n F e R h , f o r e x a m p l e , a n e x t e r n a l m a g n e t i c f i e l d f o r c i b l y a l i g n s t h e F e l o c a l s p i n s , a n d e v e n t u a l l y i n d u c e s a t r a n s i t i o n f r o m t h e a n t i f e r r o m a g n e t i c g r o u n d s t a t e t o t h e f e r r o m a g n e t i c s t a t e a t a r o u n d r o o m t e m p e r a t u r e . S i n c e p h o t o - i r r a d i a t i o n s e l e c t i v e l y e x c i t e s m o l e c u l a r u n i t s i n s o l i d s y s t e m s , w e c a n e x p e c t a n e w t y p e p h a s e t r a n s i t i o n w h o s e o r d e r p a r a m e t e r i s t h e d e n s i t y o f t h e e x c i t e d m o l e c u l e s . I n F i g . 1 , w e s h o w s c h e m a t i c p i c t u r e s o f a m o l e c u l a r s y s t e m u n d e r p h o t o - e x c i t a t i o n : o p e n a n d f i l l e d c i r c l e s r e p r e s e n t t h e m o l e c u l a r u n i t s i n t h e g r o u n d a n d e x c i t e d s t a t e s , r e s p e c t i v e l y . U n d e r w e a k p h o t o - e x c i t a t i o n , t h e p r o c e s s o f c r e a t i n g t h e e x c i t e d m o l e c u l e s b a l a n c e s t h e t h e r m a l a n d / o r q u a n t u m r e l a x a t i o n p r o c e s s e s a c t i n g u p o n t h e m . W h a t h a p p e n s i f w e f u r t h e r i n c r e a s e t h e e x c i t a t i o n p o w e r ? I n t h i s c a s e , t h e i n t e r a c t i o n b e t w e e n t h e e x c i t e d m o l e c u l e s w i l l s t a b i l i z e e i t h e r t h e e x c i t e d m o l e c u l a r p a i r s o r t h e d e n s e r e g i o n s o f e x c i t e d m o l e c u l e s ( h a t c h e d r e g i o n i n F i g . 1 ) . I n o t h e r w o r d s , t h e l i f e t i m e o f t h e d e n s e r e g i o n b e c o m e s m u c h l o n g e r t h a n t h a t o f t h e i s o l a t e d e x c i t e d m o l e c u l e s . C o n s e q u e n t l y , s u c h r e g i o n s t e n d t o g r o w b y t h e s u b s e q u e n t p h o t o - e x c i t a t i o n s , a n d e v e n t u a l l y t h e e x c i t e d m o l e c u l e s c o n d e n s e . T o e x p e r i m e n t a l l y c o n f i r m t h e a b o v e - m e n t i o n e d s c e n a r i o , w e h a v e p e r f o r m e d i n s i t u s y n c h r o t r o n r a d i a t i o n X - r a y ( S R X ) p o w d e r s t r u c t u r a l a n a l y s i s o f [ F e ( p t z ) 6 ] ( B F 4 ) 2 u n d e r p h o t o - e x c i t a t i o n a t b e a m l i n e B L 0 2 B 2 [ 1 ] . [ F e ( p t z ) 6 ] ( B F 4 ) 2 ( p t z = p r o p y l t e t r a z o l e ) , c o n s i s t s o f F e ( p t z ) 6 m o l e c u l a r u n i t s , i n w h i c h a d i v a l e n t F e 2 + i o n i s s u r r o u n d e d b y s i x N o f p t z l i g a n d s . T h e F e ( p t z ) 6 u n i t t a k e s t w o w e l l - d e f i n e d s t a t e s ; o n e i s t h e l o w - s p i n ( L S ) g r o u n d s t a t e , a n d t h e o t h e r o n e i s t h e h i g h - s p i n ( H S ) e x i t e d s t a t e . T h e F e - N b o n d d i s t a n c e ( 2 . 2 Å ) i s l a r g e r i n t h e H S s t a t e t h a n t h a t ( 2 . 0 Å ) i n t h e L S s t a t e . F i g u r e 2 s h o w s t h e p o w d e r p a t t e r n s f o r [ F e ( p t z ) 6 ] ( B F 4 ) 2 a t 9 1 K u n d e r v a r i o u s p h o t o - e x c i t a t i o n c o n d i t i o n s . A c w l a s e r ( 5 3 2 n m ) b e a m , w h i c h s e l e c t i v e l y e x c i t e s t h e L S m o l e c u l e s i n t o t h e H S s t a t e , w a s w e a k l y f o c u s e d o n t o t h e p o w d e r e d ( a ) W e a k E x c i t a t i o n ( b ) S t r o n g E x c i t a t i o n F i g . 1 . S c h e m a t i c p i c t u r e s o f a m o l e c u l a r s y s t e m u n d e r ( a ) w e a k a n d ( b ) s t r o n g e x c i t a t i o n . O p e n a n d f i l l e d c i r c l e s r e p r e s e n t t h e m o l e c u l a r u n i t s i n t h e g r o u n d a n d e x c i t e d s t a t e s , r e s p e c t i v e l y . H a t c h i n g i n ( b ) i n d i c a t e s t h e d e n s e r e g i o n o f t h e e x c i t e d m o l e c u l e s . 31 F i g . 2 . I n s i t u s y n c h r o t r o n - r a d i a t i o n X - r a y p o w d e r d i f f r a c t i o n p a t t e r n s u n d e r v a r i o u s e x c i t a t i o n p o w e r s f o r [ F e ( p t z ) 6 ] ( B F 4 ) 2 a t 9 1 K . X - r a y w a v e l e n g t h i s 1 . 0 0 1 0 Å . T h e t h i c k s o l i d c u r v e i s t h e p o w d e r p a t t e r n w i t h o u t p h o t o - e x c i t a t i o n . T h e b r o k e n c u r v e i s t h e p o w d e r pattern at 150 K in the high-temperature phase. Fig. 3 Fig. 3(a) Figure 3(b) F i g u r e 3 ( c ) Fig. 4 s a m p l e s i n a c a p i l l a r y ; t h e l a s e r s p o t s i z e ( c a . 2 mm ) wa s la rg er th an th e co ll im at or si ze (1 mm × 0.5 mm ) for the SRX beam. The powder patterns s h o w e d n e g l i g i b l e c h a n g e i n t h e w e a k e x c i t a t i o n region ( P < 49 mW ; P is the excitation power), and were analyzed with the rhombohedral ( R3 ; Z = 3) model [2]. At P = 55 mW and 70 mW, however, the B r a g g r e f l e c t i o n a t 1 9 . 2 s h o w e d a s h o u l d e r structure in the lower-angle side. As a result , we adopted a two-phase model with the rhombohedral s t r u c t u r e s t o d e t e r m i n e t h e R i e t v e l d a n a l y s i s o f these patterns. I n w e summarized the P -dependence of physical quantities for [Fe( ptz ) 6 ]( BF 4 ) 2 at 91 K. The lattice constants ( ) gradually change with P in the weak excitation region ( P < 49 mW ). At P = 5 5 m W , h o w e v e r , t h e s e c o n d a r y p h a s e ( s q u a r e symbols) suddenly appears. The l attice constant a (10.82 - 10.83 Å) for this secondary phase is much smaller than the high-temperature value (10.89 Å), i n d i ca ti n g th a t th e p re se n t o b se rv a ti o n ca n n o t b e ascribed to the conventional heating effects. These s t r u c t u r a l c h a n g e s a r e m a i n t a i n e d o n l y u n d e r p h o t o - e x c i t a t i o n , a n d s o o n d i s a p p e a r i f w e s t o p it . shows the P -dependence of t h e F e - N b o n d d i s t a n c e d Fe -N d e t e r m i n e d b y t h e R i e t v e l d s t r u c t u r a l a n a l y s i s . T h e l o n g e r d F e - N ( 2 . 1 Å ) s u g g e s t s t h a t t h e secondary phase should be ascribed to the condensed phase of the photo-excited HS m o l e c u l e s . s h o w s t h e m a s s fraction of the secondary phase. Now, let us investigate the interrelation be tw ee n P an d th e or de r pa ra me te r, th at is, density n HS of the HS molecules, which c a n b e e s t i m a t e d b y v a r y i n g t h e L S absorption band [3]. I n , w e plotted n HS against P for [Fe( ptz ) 6 ]( BF 4 ) 2 at 77 K ; so li d an d op en ci rc le s re pr es en t th e da ta o b t a i n e d i n P - i n c r e a s i n g a n d d e c r e a s i n g runs, respectively. We used the 514.5 nm ( 2 . 4 1 e V ) l i n e o f a c w A r + l a s e r a s t h e excitation and probe light sources (pump & p r o b e t e c h n i q u e ) . I n t h e w e a k - e x c i t a t i o n r e g i o n ( P < 6 0 m W / c m 2 ) , n H S r e m a i n s small (< 0.1). With a further increase in P , h o w e v e r , n H S s t e e p l y i n c r e a s e s a n d reaches ca. 0.5. Such an abrupt change of n H S a g a i n s t P c a n b e r e g a r d e d a s t h e ph as e tr an si ti on in to th e co nd en se d st at e of the HS molecules. We further observed a c l e a r o p t i c a l h y s t e r e s i s , i n d i c a t i n g t h a t this phase transition is of the first order. 32 1 8 1 9 2 0 2 1 2 θ ( d e g . ) 1 8 m W 2 8 m W 1 2 m W 3 9 m W 4 9 m W 5 5 m W 7 0 m W 0 m W 1 5 0 K I n t e n s i t y ( a r b . u n i t s ) [ F e ( p t z ) 6 ] ( B F 4 ) 2 a t 9 1 K F i g . 3 . E x c i t a t i o n p o w e r d e p e n d e n c e o f ( a ) l a t t i c e c o n s t a n t s , a a n d c , ( b ) F e - N b o n d distance d Fe-N and (c) mass fraction v s of the secondary phase for [Fe( ptz ) 6 ]( BF 4 ) 2 at 91 K. Square symbols represent values for the photo- induced secondary phase. Fig. 4. Density n HS of the HS molecules a g a i n s t e x c i t a t i o n p o w e r d e n s i t y P f o r [Fe( ptz ) 6 ]( BF 4 ) 2 at 77 K. References [ 1 ] Y u t a k a M o r i t o m o , K . K a t o , A . K u r i k i , A . N a k a m o t o , N . K o j i m a , M . T a k a t a a n d M . Sakata, J. Phys. Soc. Jpn. 71 (2002) 2609. [2] Yutaka Moritomo K. Kato, A. Nakamoto, N. Kojima, E. Nishibori, M. Takata and M. Sakata, J. Phys. Soc. Jpn. 71 (2002) 1015. [ 3 ] X . J . L i u , Y . M o r i t o m o , T . K a w a m o t o , A . N a k a m o t o a n d N . K o j i m a , P h y s . R e v . 6 7 (2003) 012102. T h u s , o u r i n s i t u S R X p o w d e r s t r u c t u r a l analysis , in addition to results of the pump & probe e x p e r i m e n t o f [ F e ( p t z ) 6 ] ( B F 4 ) 2 , s u g g e s t s t h a t c o n d e n s a t i o n o f e x c i t e d m o l e c u l e s i s r e a l i z e d u n d e r s t r o n g - e x c i t a t i o n . P r e s e n t i n s i t u S R X p o w d e r s t r u c t u r a l a n a l y s i s i s a p o w e r f u l t o o l f o r d e t e r m i n i n g t h e a t o m i c c o o r d i n a t e s i n t h e p h o t o - excited steady state, and for explor ing the physics of excited state s. Yutaka Moritomo a , Kenichi Kato b and XiaoJun Liu a (a) Nagoya University (b) SPring-8 / JASRI E-mail: moritomo @ nano.nuap.nagoya-u.ac.jp 33 0 2 0 4 0 6 0 8 0 1 0 0 P o w e r ( m W ) ν s d F e - N ( Å ) a ( Å ) c ( Å ) [ F e ( p t z ) 6 ] ( B F 4 ) 2 a t 9 1 K ( a ) ( b ) ( c ) 0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 9 2 2 . 1 2 . 2 1 0 . 6 1 0 . 7 1 0 . 8 1 0 . 9 3 2 . 0 3 2 . 2 3 2 . 4 3 2 . 6 C i r c l e : m a i n p h a s e S q u a r e : s e c o n d a r y p h a s e [ F e ( p t z ) 6 ] ( B F 4 ) 2 a t 7 7 K P ( m W / c m 2 ) 0 1 0 0 2 0 0 0 . 0 0 . 5 n H S
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