VOLUME 88, NUMBER 17 PHYSICAL REVIEW LETTERS 29 APRIL 2002 Measurement of Transition Dipole Moments in Lithium Dimers Using Electromagnetically Induced Transparency J. Qi, 1, * F. C. Spano, 1 T. Kirova, 1 A. Lazoudis, 1 J. Magnes, 1 L. Li, 2 L. M. Narducci, 3 R. W. Field, 2,4 and A. M. Lyyra 1, † 1 Departments of Physics and Chemistry, Temple University, Philadelphia, Pennsylvania 19122 2 Department of Physics, Tsinghua University, Beijing, China 100084 3 Department of Physics, Drexel University, Philadelphia, Pennsylvania 19104 4 Chemistry Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 27 September 2001; published 15 April 2002) We have observed electromagnetically induced transparency in a Doppler broadened molecular cascade system using fluorescence detection. We demonstrate that the power-dependent splitting of lines in the upper-level fluorescence excitation spectrum can be used as a new spectroscopic tool for the measurement of molecular transition dipole moment functions. DOI: 10.1103/PhysRevLett.88.173003 PACS numbers: 33.40. +f, 42.50.Hz Coherence phenomena in laser-atom interactions have been a focus of interest for decades, beginning with Fano’s pioneering studies [1]. Coherent population trapping [2], electromagnetically induced transparency (EIT) [3], lasing without inversion [4], and ultraslow propagation of light [5], among others, have been predicted and observed in atomic systems. Fewer experimental studies have addressed coherence phenomena in molecular systems [6] and, in particular, the possible occurrence of EIT [7]. This is perhaps due to the small size of typical molecular transition dipole moments. In addition, unlike atoms, even the simplest molecules are open systems in that every excited molecular rovibrational level is radiatively coupled to many more energy levels than any atomic excited state. Therefore, coherence effects in molecular systems are more challenging in terms of both experimental observation and development of theoretical analyses. In a previous paper [8] we emphasized that the Autler- Townes (AT) splitting can be used in a four-level system as a way to facilitate all-optical control of molecular angular momentum alignment. We also demonstrated that molecu- lar transition dipole moments can be measured through AT splitting, as done, for example, by Quesada et al. in pulsed laser experiments on the H 2 molecule [9]. Thus, coherence effects may allow measurement of important molecular pa- rameters. In this Letter we show that EIT can be observed even without sub-Doppler resolution using two frequency stabilized tunable lasers in a three-level system. We also demonstrate the use of this coherent effect to measure the transition dipole moment matrix element between two of the excited molecular levels using a much less demanding experimental arrangement than in [8]. A characteristic signature of EIT for the system shown in Fig. 1 is the enhanced transmission of a weak probe nearly resonant with the j1 !j2 transition, in the pres- ence of a strong coupling field resonant with the j2 !j3 transition. EIT, however, can also be recognized by the appearance of a sharp dip in the fluorescence excitation spectrum of the intermediate level [10], under resonance conditions for the probe. The connection between this fea- ture and EIT can be best understood if we consider that a cascade system becomes formally equivalent to a lambda system after moving its topmost state to a position below the middle level [3(c)]. It is well known that, under EIT conditions, the population of the highest energy level of a lambda system displays a dip, which signals the emergence of a dark state. The same holds true for the population of the middle level of a cascade system. The fluores- cence from the highest level (3) is also affected by the coupling field, especially if this field is sufficiently strong. The upper-level excitation spectrum, obtained by scanning the probe laser while holding the coupling laser on reso- nance and monitoring (filtered) side fluorescence from the L 2 |1 > |2 |3 L 1 ) 14 , 11 ( 1 g G ) 14 , 12 ( 1 u A ) 14 , 13 ( 1 u A ) 15 , 4 ( 1 g + X ) 13 , 4 ( 1 g + X L 2 |2 > |3 > L 1 ) 14 , 11 ( 1 g G ) 14 , 12 ( 1 u + ) 14 , 13 ( u + A Π Σ Σ Σ Σ FIG. 1. 7 Li 2 three-level cascade scheme: The weak probe laser, L 1 (15642.636 cm 21 ), was used to excite molecules from the ground state level X 1 S 1 g y 1  4, J 1  15 to an excited intermediate level A 1 S 1 u y 2  13, J 2  14. The laser, L 2 (17053.954 cm 21 ), resonantly coupled the intermediate level to a higher electronic state level G 1 P g y 3  11, J 3  14, f . 173003-1 0031-9007 02 88(17) 173003(4)$20.00 © 2002 The American Physical Society 173003-1