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7.%20Atomic and Molecular Physics NET-JRF%20June%202011-Dec%202014

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Brijmohan
Banaras Hindu University (BHU), Varanasi
PhD Condensed matter Physics

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fiziks Institute for NET/JRF, GATE, IIT-JAM, JEST, TIFR and GRE in PHYSICAL SCIENCES ATOMIC AND MOLECULAR PHYSICS NET/JRF (JUNE-2011) Q1. Consider the energy level diagram (as shown in the figure below) of a typical three level ruby laser system with 1.6 1019 Chromium ions per cubic centimeter. All the atoms excited by the 0.4 m radiation decay rapidly to level E2 which has a lifetime = 3 ms. E3 0.4 m 0.7 m E2 E1 A. Assuming that there is no radiation of wavelength 0.7 m present in the pumping cycle and that the pumping rate is R atoms per cm3, the population density in the level N2 builds up as: (a) N2 (t) = R (et/ 1) (c) N 2 t Ans: Rt 2 (b) N2 (t) = R (1 e-t/ ) 1 e t / (d) N2 (t) = R t (b) N 2 t R 1 e t / B. The minimum pump power required (per cubic centimeter) to bring the system to transparency, i.e. zero gain, is (a) 1.52 kW (b) 2.64 kW (c) 0.76 kW (d) 1.32 kW Ans: (c) Solution: The Minimum Power required to achieve zero gain is P N hv N hc 1.6 10 19 6.6 10 34 3 10 8 754 W cm 3 6 3 2 0.7 10 3 10 2 2 P 0.76kW per cubic centimeter Head office fiziks, H.No. 23, G.F, Jia Sarai, Near IIT, Hauz Khas, New Delhi-16 Phone: 011-26865455/+91-9871145498 Website: www.physicsbyfiziks.com Email: fiziks.physics@gmail.com Branch office Anand Institute of Mathematics, 28-B/6, Jia Sarai, Near IIT Hauz Khas, New Delhi-16 1

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