Adding Custom Permission level to Document Library using Powershell

Hi All,

I have some powershell scripts to create document library with Permissions, and it works well. But I have a custom Permission levels like (Contribute Restricted). So when I replace the "Contribute" to "Contribute Restricted" in the below code, It doesn't create that permission. Please can some one help me adding custom permission levels toRoleDefinitions in Powershell

    $group = $web.SiteGroups["Admin Members"]
    $roleAssignment = new-object Microsoft.SharePoint.SPRoleAssignment($group)
    $roleDefinition = $web.RoleDefinitions["Contribute"];
    $roleAssignment.RoleDefinitionBindings.Add($roleDefinition);
    $list.RoleAssignments.Add($roleAssignment)