Contents

Energy Unit Conversions (energy)

Module: utils_energy

Overview

Provides functions to convert between different energy and wavelength units used in X-ray spectroscopy. Uses CODATA 2018 physical constants for high precision.

Key Features

  • Convert between 7 different units: keV, eV, Rydberg, Joules, Hz, Angstroms, nanometers

  • High precision calculations using double precision arithmetic

  • Handles edge cases (e.g., very small wavelengths)

  • Based on CODATA 2018 physical constants

Supported Units

Unit Code

Full Name

Description

'kev'

Kiloelectron volts

Standard X-ray energy unit

'ev'

Electron volts

1/1000 of keV

'ryd'

Rydberg

Atomic energy unit

'j'

Joules

SI energy unit

'hz'

Hertz

Frequency unit

'ang'

Angstroms

Wavelength unit (10⁻¹⁰ m)

'nm'

Nanometers

Wavelength unit (10⁻⁹ m)

API Reference

Constants

Conversion Factors

Constant

Value

Description

kevtohz

2.41798924208492e17_dp

keV to Hz (e/h, CODATA 2018)

kevtoang

12.39841984332_dp

keV to Angstroms (hc/e, CODATA 2018)

rydtokev

0.013605693122994_dp

Rydberg to keV

ktokev

8.61733326214518e-8_dp

Kelvin to keV

Functions

calc_energy

calc_energy(x1, x2, unit1, unit2)

Convert energy/wavelength between different units

Parameters:

  • x1 – Input value (real(dp))

  • x2 – Output value (real(dp))

  • unit1 – Input unit (character*3)

  • unit2 – Output unit (character*3)

Returns:

x2 contains converted value

Converts from unit1 to unit2 through keV as intermediate representation.

Usage Examples

Basic Energy Conversion

use utils_energy
use utils_cspex, only: dp

real(dp) :: energy_kev, wavelength_ang, frequency_hz

! Convert 1 keV to Angstroms
call calc_energy(1.0_dp, wavelength_ang, 'kev', 'ang')
write(*,*) '1 keV = ', wavelength_ang, ' Angstroms'

! Convert 1 keV to Hz
call calc_energy(1.0_dp, frequency_hz, 'kev', 'hz')
write(*,*) '1 keV = ', frequency_hz, ' Hz'

Wavelength to Energy

use utils_energy
use utils_cspex, only: dp

real(dp) :: energy_kev, wavelength_nm

! Convert 10 nm to keV
wavelength_nm = 10.0_dp
call calc_energy(wavelength_nm, energy_kev, 'nm', 'kev')
write(*,*) wavelength_nm, ' nm = ', energy_kev, ' keV'

Unit Round-Trip

use utils_energy
use utils_cspex, only: dp

real(dp) :: original_kev, converted_kev, temp_ang

original_kev = 6.4_dp  ! Fe K-alpha line

! Convert keV to Angstroms and back
call calc_energy(original_kev, temp_ang, 'kev', 'ang')
call calc_energy(temp_ang, converted_kev, 'ang', 'kev')

write(*,*) 'Original: ', original_kev, ' keV'
write(*,*) 'Round-trip: ', converted_kev, ' keV'
write(*,*) 'Difference: ', abs(original_kev - converted_kev)

Common X-ray Lines

use utils_energy
use utils_cspex, only: dp

real(dp) :: energy_kev, wavelength_ang

! Fe K-alpha line (6.4 keV)
call calc_energy(6.4_dp, wavelength_ang, 'kev', 'ang')
write(*,*) 'Fe K-alpha: 6.4 keV = ', wavelength_ang, ' Angstroms'

! O K-alpha line (0.525 keV)
call calc_energy(0.525_dp, wavelength_ang, 'kev', 'ang')
write(*,*) 'O K-alpha: 0.525 keV = ', wavelength_ang, ' Angstroms'

Notes

Precision Considerations

  • All calculations use double precision (dp) for maximum accuracy

  • Uses tiny(x) to avoid division by zero for very small wavelengths

  • CODATA 2018 constants ensure scientific accuracy

Performance

  • Simple arithmetic operations - very fast

  • No memory allocation - suitable for tight loops

  • Minimal branching - good for modern CPUs

Error Handling

  • No explicit error checking for invalid unit codes

  • Invalid units may produce incorrect results

  • Consider adding validation for production code

Best Practices

  1. Always use _dp suffix for literal values

  2. Check that output values are reasonable

  3. For critical applications, validate unit codes

  4. Use intermediate variables for complex conversions

See Also