ADAS Subroutine cxqxn

      SUBROUTINE CXQXN ( MXNENG , MXNSHL , MXBEAM , NBEAM  ,
     &                   BMENA  , BMFRA  , NBOT   , NTOP   ,
     &                   NMINF  , NMAXF  , NENRGY , ENRGYA ,
     &                   ALPHAA , XSECNA , QTHEOR , RATE
     &                 )
C
C
C-----------------------------------------------------------------------
C
C  ****************** FORTRAN77 SUBROUTINE: CXQXN  *********************
C
C  PURPOSE:  USES THE INPUT DATASET TO CALCULATE THE CHARGE EXCHANGE
C            RATE COEFFICIENTS FOR N-LEVELS AVERAGED OVER THE BEAM
C            FRACTIONS.
C
C  CALLING PROGRAM: ADAS308
C
C  INPUT : (I*4)  MXNENG    = MAXIMUM NO. OF ENERGIEL.
C  INPUT : (I*4)  MXNSHL    = MAXIMUM NO. OF N SHELLS.
C  INPUT : (I*4)  MXBEAM    = MAXIMUM NO. OF BEAM ENERGIES.
C  INPUT : (I*4)  NBEAM     = NUMBER OF BEAM ENERGIES.
C  INPUT : (R*8)  BMENA()   = BEAM ENERGY COMPONENTS.
C                             UNITS: EV/AMU
C                             DIMENSION: COMPONENT INDEX.
C  INPUT : (R*8)  BMFRA()   = BEAM COMPONENT FRACTIONS.
C                             DIMENSION: COMPONENT INDEX.
C  INPUT : (I*4)  NBOT      = MINIMUM PRINCIPAL QUANTUM NUMBER.
C  INPUT : (I*4)  NTOP      = MAXIMUM PRINCIPAL QUANTUM NUMBER.
C  INPUT : (I*4)  NMINF     = MINIMUM PRINCIPAL QUANTUM NUMBER OF INPUT
C                             DATASET.
C  INPUT : (I*4)  NMAXF     = MAXIMUM PRINCIPAL QUANTUM NUMBER OF INPUT
C                             DATASET.
C  INPUT : (I*4)  NENRGY    = NUMBER OF ENERGIES IN DATASET.
C  INPUT : (R*8)  ENRGYA()  = COLLISION ENERGIES.
C                             UNITS: EV/AMU
C                             DIMENSION: ENERGY INDEX
C  INPUT : (R*8)  ALPHAA()  = EXTRAPOLATION PARAMETER ALPHA.
C                             DIMENSION: ENERGY INDEX
C  INPUT : (R*8)  XSECNA(,) = N-RESOLVED CHARGE EXCHANGE CROSS-SECTIONS.
C                             UNITS: CM2
C                             1ST DIMENSION: ENERGY INDEX
C                             2ND DIMENSION: N-SHELL
C
C  OUTPUT: (R*8)  QTHEOR()  = MEAN RATE COEFFICIENTS FOR N-LEVELS
C                             AVERAGED OVER BEAM FRACTIONS.
C                             UNITS: CM3 SEC-1
C                             DIMENSION: REFERENCED BY N QUANTUM NUMBER.
C  OUTPUT: (R*8)  RATE(,)   = RATE COEFFICIENTS FOR EACH COMPONENT OF
C                             THE BEAM AS A FUNCTION OF N-LEVEL.
C                             UNITS: CM3 SEC-1
C                             1ST DIMENSION: BEAM INDEX
C                             2ND DIMENSION: N-SHELL
C
C  PARAM : (R*8)  ALPMIN    = MINIMUM VALUE OF EXTRAPOLATION EXPONENT
C                             'ALPHA'.
C  PARAM : (R*8)  C1        =
C
C          (I*4)  IB        = BEAM INDEX.
C          (I*4)  N         = PRINCIPAL QUANTUM NUMBER.
C          (I*4)  IE        = ENERGY INDEX.
C          (I*4)  IEU       = ENERGY INDEX ABOVE BEAM ENERGY.
C          (I*4)  IEL       = ENERGY INDEX BELOW BEAM ENERGY.
C
C          (R*8)  XNMAXF    = REAL VALUE = NMAXF.
C          (R*8)  ALPHA     = EXTRAPOLATION EXPONENT.
C          (R*8)  VEL       = VELOCITY OF INCIDENT ATOM.
C                             UNITS: CM SEC-1
C          (R*8)  GRAD      = GRADIENT OF STRAIGHT LINE INTERPOLATION.
C          (R*8)  XSEC      = CROSS-SECTION AT INTERMEDIATE ENERGY.
C
C          (L*4)  LMATCH    = FLAG IF BEAM ENERGY MATCHES EXACTLY A DATA
C                             SET ENERGY.
C                           = .TRUE.  => BEAM ENERGY MATCHES.
C                           = .FALSE. => BEAM ENERGY DOES NOT MATCH.
C
C ROUTINES:  NONE
C
C AUTHOR:  JONATHAN NASH (TESSELLA SUPPORT SERVICES PLC)
C          K1/0/81
C          JET EXT. 5183
C
C DATE:    18/10/93
C
C-----------------------------------------------------------------------
C
C-----------------------------------------------------------------------
      INTEGER             MXBEAM,      MXNENG,      MXNSHL,      NBEAM
      INTEGER             NBOT,        NENRGY,      NMAXF,       NMINF
      INTEGER             NTOP
      REAL*8              ALPHAA(MXNENG),           BMENA(MXBEAM)
      REAL*8              BMFRA(MXBEAM),            ENRGYA(MXNENG)
      REAL*8              QTHEOR(MXNSHL),           RATE(MXBEAM,MXNSHL)
      REAL*8              XSECNA(MXNENG,MXNSHL)