在发动机气缸内进行激光诱导荧光（LIF）测量时对激光吸收的一种修正方法

A new correction method for laser absorption in LIF measurements inside engine cylinders is proposed. Frequently we have a problem of absorption of excitation laser by high-density molecules in LIF measurements at high
concentration media such as in-cylinder gas. We developed a method to correct the attenuation effect by use of two counter propagating laser beams. Corrected information on molecule distribution is obtained by taking square root of data after multiplying two data which were from measurements by each excitation beam. A couple of laser light pulse A and B propagating in counter directions is considered. Pulse A is propagating in +x
direction and pulse B is going in -x direction. Pulse A induces a fluorescence light SA,
( ) ()



= Φ = −
x
A A A S n BI x y y d
0
η 0, exp α ξ , ξ (1)
where we considered laser light absorption using Lambert-Beer's law. Here,ΦA, n, η, B, IA and α(x,y) are detection
efficiency, density of the molecule, quantum yield which includes ro-vibronic state population, Einstein's B coefficient, excitation laser light intensity, and absorption coefficient (=σ×n(x,y), σ is absorption cross section). Similarly, fluorescence signal induced by pulse B is described as ( ) ()



= Φ = 
x
B B B S n BI x y y d
0
η 0, exp α ξ , ξ (2)
Multiplying and taking square root of these two equations, we obtain ( ) ()
( ) ()I x y I x y
S x y S x y
B
n
A B
A B
A B 0, 0,
1 , ,
Φ Φ = =
=
η
(3)
This Eq. (3) does not include α. This means that effect from laser absorption was canceled out. It is emphasized that α does not have to be constant, but can be a function of space. That is, this method is applicable to any fields including heterogeneous fields in principle. Eq. (3) means that we need intensity profiles of IA and IB at x=0.
The origin for x-axis can be determined freely, but IA and IB must be known at a same position. Namely, if IA was measured at the location before incidence to the measurement field (as described in Fig. 1), IB has to be measured after it propagated through the measurement field ( after the attenuation). In order to conduct absolute measurements, one will need η(x,y) which includes quantum yield and statistical ro-vibronic population determined by temperature. We should note that re-absorption of fluorescence light is not counted into this method, and generally we should be
careful on this issue. However, in case of LIF of ketone, which we will demonstrate later in this paper, the
fluorescence spectrum is shifted to region from 350 to 450 nm because of vibrational relaxation inside S1 state and it
does not overlap with absorption spectrum which located from 250 to 310 nm. This means that re-absorption of fluorescence light does not occur in case of ketone LIF. Another method using two counter propagating laser beams had been presented by Versluis et al [1]. in 1997. Our
method is totally new one and is physically different from the method proposed by Versluis et al..
In this paper, differences between Versluis' method and our method are discussed, and it is clarified that our method has a considerable advantage in case of noisy measurement fields. We also demonstrate a fuel distribution
measurement by our new method.