As a transform-limited light source

we used a pulsed Rh6G dye laser synchronously pumped by a mode locked Ar

+

lasar with

a repetition rate of 82 MHz. The operation wavelength was 514.5nm. The dye laser was tuned to 590 nm which was nearly the peak of the absorption spectrum of cresyl violet in methanol solution. The single mode-locked pulse duration

t

p

was 3 ps. The output beam of dye laser was split into two beams or the pump beam and the probe beam. The ratio of intensity between them was about 100:1. The light-spot-rodius ratio of the pump beam to the probu was 3:1. It means that the probe beam fell inside the pump beam on the sample. The energy change in the probe boam induced by the pump beam was selectively picked up by a lock-in amplifier.The dynamic equation to describe the absorption racovoiy process can be written as follows dN

3

(t)/dt=B

&

(t)N

0

-N

3

(t)/τ

3

(1) dN

2

(t)/dt=N

3

(t)/τ

32

-N

2

(t)/τ

21

(2) where N

0

is the population in the ground state at t= 0. N

3

(t) and N

2

(t) arc the transient population in |3

>

and|2

>

state

respectively. B is a constant relating to the absorption cross suction σ

13

. Wo resolved eqs. (1) and (2) using N

3

(0)=N

2

(0)=0

then N

2

(t)∝expτ

21

) can bo derived. Fitting the experimental data (curve a sea Fig. 4) with those calculated by eq. (3) (curve b)

we got the absorption recovery time τ

21

=162 ps.