Pulse train induced rotational excitation and orientation of a polar molecule Ashish Tyagi a , Urvashi Arya b,⇑ , Bhavna Vidhani c , Vinod Prasad a a Department of Physics, Swami Shraddhanand College, University of Delhi, Delhi 110036, India b Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India c Department of Physics, Hansraj College, University of Delhi, Delhi 110007, India highlights  Rotational excitation and orientation of a polar molecule is studied in presence of pulse train of different shape envelopes.  Both rotational excitation and orientation show strong dependence on shape of the pulse train.  Rotational dynamics and orientation show strong dependence on the shape of the pulse train. graphical abstract The figure shows the effect of thirteen pulse train (Gaussian shape) on the population dynamics of ground and excited rotational states of HBr molecule. Inset shows the orientation of the molecule due to single pulse. article info Article history: Received 30 December 2013 Received in revised form 27 February 2014 Accepted 15 March 2014 Available online 1 April 2014 Keywords: Rotational excitation Orientation Pulse train Ladder system abstract We investigate theoretically the rotational excitation and field free molecular orientation of polar HBr molecule, interacting with train of ultrashort laser pulses. By adjusting the number of pulses, pulse period and the intensity of the pulse, one can suppress a population while simultaneously enhancing the desired population in particular rotational state. We have used train of laser pulses of different shaped pulse envelopes. The dynamics and orientation of molecules in the presence of pulse train of different shapes is studied and explained. Ó 2014 Elsevier B.V. All rights reserved. Introduction Population transfer to a desired coherent superposition of atomic and molecular states (i.e., a wave packet) has been a major goal during the past three decades and continues to be a challenge, for instance, for implementation of chemical and biological pro- cesses [1,2], for fast quantum information processing [3], and for nonlinear optics [4]. Since the invention of the mode-locked lasers, pulse trains have been used in the measurement of spectroscopy [5]. The generated single pulse in the train can be of attosecond duration [6] or of few-cycle oscillations [7]; the repetition fre- quency of the pulse train can be in the THz regime [8] and it can http://dx.doi.org/10.1016/j.saa.2014.03.075 1386-1425/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +91 9911979305. E-mail addresses: ashishtyagi.du@gmail.com (A. Tyagi), urvashivarsheney@ yahoo.co.in (U. Arya), bhavna.vidhani@gmail.com (B. Vidhani), vprasad@ss.du.ac.in (V. Prasad). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 193–200 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa